diff --git a/UnitTestProject/Modules/SwapModule/Accrual/SimplePeriodShadowTest.cs b/UnitTestProject/Modules/SwapModule/Accrual/SimplePeriodShadowTest.cs
new file mode 100644
index 00000000..36f6d3ac
--- /dev/null
+++ b/UnitTestProject/Modules/SwapModule/Accrual/SimplePeriodShadowTest.cs
@@ -0,0 +1,150 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Newtonsoft.Json;
+using YLErp;
+using YLErp.DBModels;
+using YLErp.DBModels.Enums;
+using YLErp.Modules.SwapModule;
+using YLErp.Modules.SwapModule.Accrual;
+using YLErp.Derivatives.Interest;
+
+namespace UnitTestProject.Modules.SwapModule.Accrual
+{
+    /// <summary>
+    /// 影子测试：CalcDailySimpleInterest（旧逐日循环）vs FundingLegAccrual.AccrueSimplePeriod（新分段纯函数）。
+    /// </summary>
+    [TestClass]
+    public class SimplePeriodShadowTest
+    {
+        private const decimal Notional = 100_000_000m;
+        private const decimal Spread = 0.0025m;
+        private const int AnnualDays = 365;
+        private static readonly DateTime StartDate = new(2026, 4, 21);
+        private static readonly DateTime EndDate = new(2026, 5, 11); // 21天, 7天周期→重置日 4/28, 5/5
+
+        private static trade CreateTrade()
+        {
+            return new trade
+            {
+                id = 1, TradeNumber = "UT-SIMPLE-SHADOW", ClientId = 999998,
+                TradeType = "收益互换", TradeDate = StartDate, StartDate = StartDate,
+                ExerciseDate = StartDate.AddYears(1), TradeStatus = "确认成交", ValidState = "Valid",
+                trade_extend = new trade_extend
+                {
+                    TradeId = 1,
+                    ExtendJson = JsonConvert.SerializeObject(new TradeExtendJson
+                    {
+                        AnnualDays = AnnualDays, InterestCalcMode = "10", SettlementRules = 0
+                    })
+                }
+            };
+        }
+
+        private static swap_position CreatePosition()
+        {
+            return new swap_position
+            {
+                id = 1001, SwapTradeId = 1, PosiDirection = 0,
+                InterestDirection = (int)SwapDirectionEnum.收取,
+                InterestMode = (int)InterestModeEnum.标的期初全价,
+                InterestRateDefault = Spread,
+                InterestPrincipalFix = Notional,
+                PosiStartDate = StartDate, PosiMatuirityDate = StartDate.AddYears(1),
+                IsInitial = true, Invalid = false,
+                InterestType = (int)InterestTypeEnum.单利,
+                IsAnnualized = true, interest_rest_days = 7, interest_rule = 0,
+                FloatRateUnderlyingCode = null,
+                InterestSwapInterval = "[]"
+            };
+        }
+
+        private sealed class StubSvc : SwapDealService
+        {
+            public StubSvc() : base(new OptUserInfo(0, nameof(SimplePeriodShadowTest), OptUserFrom.UnitTest)) { }
+        }
+
+        /// <summary>
+        /// 固定利率（无FR007）算头不算尾，全平，无历史归档。
+        /// </summary>
+        [TestMethod]
+        public void 影子_固定利率_无归档_旧新一致()
+        {
+            var td = CreateTrade();
+            var position = CreatePosition();
+            var flowEvent = new swap_flow_event { InterestRate = Spread };
+            var preEod = new eod_swap_position { id = 0, TdInterestPrincipal = 0, InterestProfitSum = 0 };
+
+            // 旧方法
+            decimal oldI = 0, oldTd = 0;
+            var svc = new StubSvc();
+            svc.CalcDailySimpleInterest(preEod, EndDate, position, Notional, flowEvent,
+                AnnualDays, false, 0m, 1m, Notional, true, false, ref oldI, ref oldTd);
+
+            // 新方法：固定利率全段相同
+            // 旧代码差分: dynomicPrincipal = preEod.TdInterestPrincipal(=0) + posiPrincipal - orginPv = 0
+            var segRates = new List<(DateTime, decimal)> { (StartDate, Spread) };
+            var accrualPrincipal = 0m + Notional - Notional; // 差分 = 0
+            var result = FundingLegAccrual.AccrueSimplePeriod(
+                priorUnrealized: 0m,
+                accrualPrincipal: 0m, // 差分=0(无归档时 preEod.TdInterestPrincipal=0)
+                closeRatio: 1m,
+                segmentRates: segRates,
+                startDate: StartDate,
+                endDate: EndDate,
+                priorValueDate: DateTime.MinValue,
+                boundary: AccrualBoundary.StartOnly,
+                annualDays: AnnualDays,
+                isAnnualized: true);
+
+            Console.WriteLine($"旧: I={oldI} Td={oldTd}");
+            Console.WriteLine($"新: Accrued={result.Accrued} AccruedToday={result.AccruedToday}");
+            Assert.AreEqual((double)oldI, (double)result.Accrued, 0.01, "InterestAmount 一致");
+        }
+
+        /// <summary>
+        /// 有历史归档（preEod.id != 0），续接上一日终。
+        /// </summary>
+        [TestMethod]
+        public void 影子_有归档_续接_旧新一致()
+        {
+            var position = CreatePosition();
+            var preEodDate = new DateTime(2026, 5, 4); // 上一日终 = 第14天
+            var preEod = new eod_swap_position
+            {
+                id = 1, SwapTradeId = 1, PositionId = 1001,
+                ValueDate = preEodDate,
+                TdInterestPrincipal = Notional,
+                InterestProfitSum = 200_000m,
+                PosiNotionalValue = Notional, FloatRate = 0m
+            };
+            var flowEvent = new swap_flow_event { InterestRate = Spread };
+
+            // 旧方法
+            decimal oldI = 0, oldTd = 0;
+            var svc = new StubSvc();
+            svc.CalcDailySimpleInterest(preEod, EndDate, position, Notional, flowEvent,
+                AnnualDays, false, 0m, 0.5m, Notional, true, false, ref oldI, ref oldTd);
+
+            // 新方法
+            // 差分本金 = preEod.TdInterestPrincipal + posiPrincipal - orginPv
+            var accrualPrincipal = Notional + Notional - Notional;
+            var segRates = new List<(DateTime, decimal)> { (StartDate, Spread) };
+            var result = FundingLegAccrual.AccrueSimplePeriod(
+                priorUnrealized: 200_000m * 0.5m, // InterestProfitSum × closePercent
+                accrualPrincipal: accrualPrincipal,
+                closeRatio: 0.5m,
+                segmentRates: segRates,
+                startDate: StartDate,
+                endDate: EndDate,
+                priorValueDate: preEodDate,
+                boundary: AccrualBoundary.StartOnly,
+                annualDays: AnnualDays,
+                isAnnualized: true);
+
+            Console.WriteLine($"旧: I={oldI} Td={oldTd}");
+            Console.WriteLine($"新: Accrued={result.Accrued} AccruedToday={result.AccruedToday}");
+            Assert.AreEqual((double)oldI, (double)result.Accrued, 0.01, "InterestAmount 一致");
+        }
+    }
+}
diff --git a/YLErpDAL/Modules/SwapModule/Accrual/FundingLegAccrual.cs b/YLErpDAL/Modules/SwapModule/Accrual/FundingLegAccrual.cs
index 9e2e6da5..183edd75 100644
--- a/YLErpDAL/Modules/SwapModule/Accrual/FundingLegAccrual.cs
+++ b/YLErpDAL/Modules/SwapModule/Accrual/FundingLegAccrual.cs
@@ -100,4 +100,75 @@ public static class FundingLegAccrual
         trace?.MarkEnd(result.Accrued, result.AccruedToday);
         return result;
     }
+
+    /// <summary>
+    /// 单利多日计息（纯函数，替换 CalcDailySimpleInterest 的"纯数学"部分）。
+    ///
+    /// 单利特征：计息本金全程恒定（差分公式 = priorAccrualPrincipal + positionPrincipal - originalPv）。
+    /// 按重置日分段，每段用对应利率算天数×日利息（无逐日循环，等价于 SwapInterest.AccrueSimple 分段累加）。
+    ///
+    /// 利率变化点由调用方通过 segmentRates 传入（已取好 FR007），本方法不取价。
+    /// </summary>
+    /// <param name="priorUnrealized">上一日终累计待实现利息（preEod.InterestProfitSum × closeRatio）。</param>
+    /// <param name="accrualPrincipal">计息本金（差分，全程恒定）。</param>
+    /// <param name="closeRatio">平仓比例。</param>
+    /// <param name="segmentRates">分段利率表：(段起日, all-in利率)，按日期升序。</param>
+    /// <param name="startDate">计息开始日（PosiStartDate）。</param>
+    /// <param name="endDate">计息结束日（平仓日）。</param>
+    /// <param name="priorValueDate">上一日终归档日（只算此日之后的利息）。</param>
+    /// <param name="boundary">算头算尾。</param>
+    /// <param name="annualDays">年化天数。</param>
+    /// <param name="isAnnualized">是否年化。</param>
+    public static InterestResult AccrueSimplePeriod(
+        decimal priorUnrealized,
+        decimal accrualPrincipal,
+        decimal closeRatio,
+        IReadOnlyList<(DateTime StartDate, decimal Rate)> segmentRates,
+        DateTime startDate,
+        DateTime endDate,
+        DateTime priorValueDate,
+        AccrualBoundary boundary,
+        int annualDays,
+        bool isAnnualized)
+    {
+        var displayPrincipal = accrualPrincipal * closeRatio;
+        decimal interest = priorUnrealized;
+        decimal tdInterest = priorUnrealized;
+        var precision = SwapInterest.FundingLegPrecision;
+
+        // 按段累加：每段内利率恒定，用 AccrualDays 算天数 × 日利息
+        var segStart = startDate;
+        var segIncludeStart = boundary.IncludeStart;
+
+        for (int si = 0; si < segmentRates.Count; si++)
+        {
+            var (segRateStart, segRate) = segmentRates[si];
+            var segEnd = si < segmentRates.Count - 1
+                ? segmentRates[si + 1].StartDate
+                : endDate;
+
+            // 跳过 priorValueDate 之前的日期（续接上一日终）
+            var effectiveStart = segStart > priorValueDate ? segStart : priorValueDate.AddDays(1);
+            if (effectiveStart > segEnd) { segStart = segEnd; continue; }
+
+            // 算头算尾：首段用 boundary.IncludeStart，后续段不算头
+            var segBoundary = AccrualBoundary.Of(segIncludeStart, segEnd == endDate && boundary.IncludeEnd);
+            var days = SwapInterest.AccrualDays(effectiveStart, segEnd, segBoundary);
+            if (days <= 0) { segStart = segEnd; segIncludeStart = false; continue; }
+
+            var dailyRate = isAnnualized ? segRate / annualDays : segRate;
+            var daily = Math.Round(displayPrincipal * dailyRate, precision, MidpointRounding.AwayFromZero);
+            var segInterest = Math.Round(daily * days, precision, MidpointRounding.AwayFromZero);
+
+            interest += segInterest;
+            tdInterest += segInterest;
+
+            segStart = segEnd;
+            segIncludeStart = false;
+        }
+
+        return new InterestResult(
+            Math.Round(interest, precision, MidpointRounding.AwayFromZero),
+            Math.Round(tdInterest, precision, MidpointRounding.AwayFromZero));
+    }
 }