diff --git a/src/Add-on/ArcBoard/ArcBoardBuild.ts b/src/Add-on/ArcBoard/ArcBoardBuild.ts
index 7d49a9d7c..d20733ab2 100644
--- a/src/Add-on/ArcBoard/ArcBoardBuild.ts
+++ b/src/Add-on/ArcBoard/ArcBoardBuild.ts
@@ -712,7 +712,7 @@ export class ArcBoardBuild
             if (this._CornerSet.has(onlyD))//坍塌
             {
                 // console.log("坍塌");
-                let p2 = this._SweepPath2.GetPointAtParam(curveIndex + param);
+                let p2 = this._SweepPath2.GetPointAtParam(curveIndex + Math.round(param));
                 derv = p2.sub(c.GetPointAtParam(param)).divideScalar(this._board.Thickness);
             }