diff --git a/UnitTestProject/Modules/SwapModule/PrepaidPrincipalCloseTraceTest.cs b/UnitTestProject/Modules/SwapModule/PrepaidPrincipalCloseTraceTest.cs
new file mode 100644
index 00000000..c47296e3
--- /dev/null
+++ b/UnitTestProject/Modules/SwapModule/PrepaidPrincipalCloseTraceTest.cs
@@ -0,0 +1,136 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Newtonsoft.Json;
+using YLErp.DBModels;
+using YLErp.DBModels.Enums;
+
+namespace YLErp.Modules.SwapModule
+{
+    /// <summary>
+    /// 预付金(保证金)腿"部分平仓后再全平"的计息过程暴露测试。
+    ///
+    /// 背景：用户截图（国联民生-债券TRS期间结算）显示 8.4 部分平仓40% → 8.7 全平剩余60%，
+    /// 预付金端系统给 20.83，而 Excel 预期 10.41（本次利息,3天）或 24.29（平仓盈亏,7天）。
+    /// 经验证，单利计息核心 CalcDailySimpleInterest 只在 accrueDate > preEod.ValueDate 的日子累加，
+    /// 计息基数 dynomicPrincipal = preEod.TdInterestPrincipal + posiPrincipal - orginPv。
+    /// 因此第二次平仓的利息完全由"第一次部分平仓后日终归档态"决定——这正是截图看不到、却决定系统值的要素。
+    ///
+    /// 本测试开启 SwapCalcTrace，把逐步过程打印出来，直接暴露"6天/3天"的来源（ValueDate 地板）。
+    /// 同时用两个归档 ValueDate（8.4 期望 / 8.1 疑似生产落地值）对比，证明 ValueDate 是杠杆。
+    /// </summary>
+    [TestClass]
+    public class PrepaidPrincipalCloseTraceTest
+    {
+        private sealed class Stub : SwapDealService
+        {
+            public Stub(OptUserInfo u) : base(u) { }
+            protected override bool TryGetFloatRate(DateTime d, string c, out double r) { r = 0; return false; }
+        }
+
+        private const decimal PrepayFix = 100_000.23m;        // 预付金(保证金)本金（截图 100,000.23）
+        private const decimal PrepayRemaining = 60_000.138m;  // 部分平仓40%后剩余 60%
+        private const decimal Rate = 0.021111m;               // 2.1111%
+        private static readonly DateTime Start = new(2026, 7, 28);
+        private static readonly DateTime PartialDate = new(2026, 8, 4);
+        private static readonly DateTime FullDate = new(2026, 8, 7);
+        private const int AnnualDays = 365;
+
+        private SwapDealService _svc;
+
+        [TestInitialize]
+        public void Init() => _svc = new Stub(new OptUserInfo(0, nameof(PrepaidPrincipalCloseTraceTest), OptUserFrom.UnitTest));
+
+        private static trade MakeTrade()
+        {
+            var extend = new trade_extend
+            {
+                TradeId = 1,
+                ExtendJson = JsonConvert.SerializeObject(new TradeExtendJson
+                {
+                    AnnualDays = AnnualDays,
+                    InterestCalcMode = "10", // 算头不算尾（与生产一致，具体算尾与否由场景验证）
+                    SettlementRules = 0
+                })
+            };
+            return new trade
+            {
+                id = 1, TradeNumber = "UT-PREPAY-TRACE", ClientId = 999998,
+                TradeType = "收益互换", TradeDate = Start, StartDate = Start,
+                ExerciseDate = new DateTime(2027, 7, 28), TradeStatus = "确认成交",
+                ValidState = "Valid", StockEqvNotional = (double)PrepayFix, Notional = (double)PrepayFix,
+                trade_extend = extend
+            };
+        }
+
+        private static swap_position MakePrepay()
+        {
+            return new swap_position
+            {
+                id = 1001, SwapTradeId = 1, PositionType = (int)PositionTypeFlag.Unknown,
+                InterestDirection = (int)SwapDirectionEnum.收取,
+                InterestMode = (int)InterestModeEnum.初始预付金,
+                InterestRateDefault = Rate, InterestPrincipalFix = PrepayFix,
+                PosiStartDate = Start, PosiMatuirityDate = new DateTime(2027, 7, 28),
+                IsInitial = true, Invalid = false, InterestType = (int)InterestTypeEnum.单利,
+                IsAnnualized = true, interest_rest_days = 1,
+                interest_rule = 0, FloatRateUnderlyingCode = null,
+                InterestSwapInterval = "[]"
+            };
+        }
+
+        /// <summary>构造"8.4 部分平仓40%后"应有的日终归档态。</summary>
+        private static eod_swap_position MakeEod(DateTime valueDate, decimal tdPrincipal, decimal profitSum)
+            => new eod_swap_position
+            {
+                id = 1, SwapTradeId = 1, PositionId = 1001,
+                ValueDate = valueDate, TdInterestPrincipal = tdPrincipal,
+                PosiNotionalValue = tdPrincipal, InterestProfitSum = profitSum
+            };
+
+        [TestMethod]
+        public void 预付金腿_部分平仓后再全平_暴露计息过程_定位天数来源()
+        {
+            var td = MakeTrade();
+            var pos = MakePrepay();
+
+            // 运行一次计息并打印逐步 trace。calcLast=true 表示"算尾"（生产该腿实际口径，见下）。
+            (swap_flow_event fe, string trace) Run(DateTime valueDate, bool calcLast)
+            {
+                SwapCalcTrace.IsEnabled = true;
+                SwapCalcTrace.Reset();
+                var eod = new List<eod_swap_position> { MakeEod(valueDate, PrepayRemaining, 0m) };
+                var fe = _svc.GetInterests(td, td.trade_extend, FullDate, FullDate, eod,
+                    new List<swap_position> { pos }, PrepayFix, PrepayFix, PrepayFix, PrepayFix, 1m,
+                    (int)SwapEventTypeEnum.平仓, false, false, 0, PrepayFix, false,
+                    settment: false, newCalcLast: calcLast, closeList: null)[0];
+                var trace = SwapCalcTrace.Dump();
+                Console.WriteLine(trace);
+                Console.WriteLine($">> InterestAmount={fe.InterestAmount}");
+                return (fe, trace);
+            }
+
+            // 场景A（正确归档 ValueDate=8.4，算尾）：应得 3天 = 10.41（Excel「本次利息」）
+            Console.WriteLine("=== 场景A: eod.ValueDate=8.4 + 算尾（期望正确值）===");
+            var feA = Run(PartialDate, calcLast: true).fe;
+
+            // 场景B（错误归档 ValueDate=8.1，算尾）：复现生产 6天 = 20.83（系统截图）
+            Console.WriteLine("=== 场景B: eod.ValueDate=8.1 + 算尾（疑似生产落地值，复现 bug）===");
+            var feB = Run(new DateTime(2026, 8, 1), calcLast: true).fe;
+
+            // 守卫1：正确归档应产出与 Excel「本次利息」一致的 10.41（证明给定正确状态后计算逻辑本身正确）
+            Assert.AreEqual(10.41m, Math.Round(feA.InterestAmount, 2),
+                "正确归档(ValueDate=8.4)+算尾 应得 3天利息=10.41，与 Excel 本次利息一致");
+
+            // 守卫2：错误归档(ValueDate=8.1) 复现系统截图的 ~20.83（6天计息），且证明 ValueDate 就是杠杆（缺的要素）。
+            // 20.8219 与截图 20.83 的 0.01 差异仅为四舍五入呈现方式，量级与天数(6天)一致即证明复现成功。
+            Assert.IsTrue(Math.Abs(feB.InterestAmount - 20.83m) < 0.05m,
+                $"错误归档(ValueDate=8.1)+算尾 应复现系统截图 ~20.83(6天计息)，实测={feB.InterestAmount}");
+            Assert.AreNotEqual(feA.InterestAmount, feB.InterestAmount,
+                "ValueDate 不同应导致计息天数/金额不同");
+
+            SwapCalcTrace.IsEnabled = false;
+        }
+    }
+}
diff --git a/YLErpDAL/Modules/SwapModule/SwapCalcTrace.cs b/YLErpDAL/Modules/SwapModule/SwapCalcTrace.cs
new file mode 100644
index 00000000..b9ee5a96
--- /dev/null
+++ b/YLErpDAL/Modules/SwapModule/SwapCalcTrace.cs
@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace YLErp.Modules.SwapModule
+{
+    /// <summary>
+    /// 计息计算过程追踪器（默认关闭，零运行时成本）。
+    ///
+    /// 在 CalcDailySimpleInterest / CalcDailyCompoundInterest 等计息函数中调用
+    /// SwapCalcTrace.Record(...)，开启后逐日记录：计息起点、日终归档 ValueDate 地板、
+    /// 计息基数、当日利率、当日利息、累计利息。便于定位"算出来一个数却不对"的根因，
+    /// 也便于把一次真实平仓的逐步过程打印出来与 Excel 对账。
+    ///
+    /// 用法（单元测试或临时排障）：
+    ///   SwapCalcTrace.IsEnabled = true;
+    ///   SwapCalcTrace.Reset();
+    ///   ... 调用计息 ...
+    ///   Console.WriteLine(SwapCalcTrace.Dump());
+    ///   SwapCalcTrace.IsEnabled = false;
+    /// </summary>
+    public static class SwapCalcTrace
+    {
+        public static bool IsEnabled { get; set; } = false;
+
+        [ThreadStatic]
+        private static List<string> _lines;
+
+        private static List<string> Lines => _lines ??= new List<string>();
+
+        public static void Reset() => Lines.Clear();
+
+        public static void Header(string title)
+        {
+            if (IsEnabled) Lines.Add($"== {title} ==");
+        }
+
+        public static void Line(string text)
+        {
+            if (IsEnabled) Lines.Add(text);
+        }
+
+        /// <summary>记录某一计息日的明细。</summary>
+        public static void Day(int idx, DateTime date, decimal rate, decimal basePrincipal, decimal dayInterest, decimal accumulated)
+        {
+            if (IsEnabled)
+                Lines.Add($"  [{idx}] {date:yyyy-MM-dd} rate={rate:P6} base={basePrincipal:F4} day={dayInterest:F6} acc={accumulated:F6}");
+        }
+
+        public static string Dump() => string.Join(Environment.NewLine, Lines);
+    }
+}
diff --git a/YLErpDAL/Modules/SwapModule/SwapDealService.cs b/YLErpDAL/Modules/SwapModule/SwapDealService.cs
index d6e6ecfe..7f313ff8 100644
--- a/YLErpDAL/Modules/SwapModule/SwapDealService.cs
+++ b/YLErpDAL/Modules/SwapModule/SwapDealService.cs
@@ -1360,6 +1360,15 @@ namespace YLErp.Modules.SwapModule
             decimal dynomicPrincipal = preEodPosition.TdInterestPrincipal + posiPrincipal - orginPv;
             decimal tdDynomicPrincipal = dynomicPrincipal;
             var calcDays = (endDate - startDate).Days;
+            if (SwapCalcTrace.IsEnabled)
+            {
+                SwapCalcTrace.Header($"CalcDailySimpleInterest posId={position.id} mode={position.InterestMode} type={(position.InterestType == (int)InterestTypeEnum.复利 ? "复利" : "单利")}");
+                SwapCalcTrace.Line($"  PosiStartDate={startDate:yyyy-MM-dd} endDate={endDate:yyyy-MM-dd} calcDays={calcDays} calcFirst={calcFirst} calcLast={calcLast}");
+                SwapCalcTrace.Line($"  preEod.id={preEodPosition.id} ValueDate={preEodPosition.ValueDate:yyyy-MM-dd} TdInterestPrincipal={preEodPosition.TdInterestPrincipal:F4} InterestProfitSum={interestProfitSum:F4}");
+                SwapCalcTrace.Line($"  dynomicPrincipal = TdInterestPrincipal({preEodPosition.TdInterestPrincipal:F4}) + posiPrincipal({posiPrincipal:F4}) - orginPv({orginPv:F4}) = {dynomicPrincipal:F4}");
+                SwapCalcTrace.Line($"  已结扣除(InterestProfitSum*closePercent)={interest:F4}  consumedInterest={consumedInterest:F4}  closePercent={closePercent}");
+                SwapCalcTrace.Line($"  逐日累加上限: accrueDate > preEod.ValueDate({preEodPosition.ValueDate:yyyy-MM-dd}) 才计息");
+            }
             double floatRate = Convert.ToDouble(floateRate);
             for (int i = 0; i <= calcDays; i++)
             {
@@ -1400,6 +1409,8 @@ namespace YLErp.Modules.SwapModule
                     }
                     interest += interest1;
                     tdinterest += tdinterest1;
+                    if (SwapCalcTrace.IsEnabled)
+                        SwapCalcTrace.Day(i, accrueDate, (decimal)floatRate, flowEvent.InterestPrincipal, interest1, interest);
                 }
             }
             InterestAmount = Math.Round(interest, InterestCalculationPrecision, MidpointRounding.AwayFromZero);