diff --git a/AMSS_NCKU_source/fdderivs_c.C b/AMSS_NCKU_source/fdderivs_c.C
index 5cc52d4..e3e9086 100644
--- a/AMSS_NCKU_source/fdderivs_c.C
+++ b/AMSS_NCKU_source/fdderivs_c.C
@@ -71,11 +71,30 @@ void fdderivs(const int ex[3],
     const double Fdxdz = F1o144 / (dX * dZ);
     const double Fdydz = F1o144 / (dY * dZ);
 
-    /* 输出清零：fxx,fyy,fzz,fxy,fxz,fyz = 0 */
+    /* 只清零主循环不覆盖的最外层边界面 */
     const size_t all = (size_t)ex1 * (size_t)ex2 * (size_t)ex3;
-    for (size_t p = 0; p < all; ++p) {
-        fxx[p] = ZEO; fyy[p] = ZEO; fzz[p] = ZEO;
-        fxy[p] = ZEO; fxz[p] = ZEO; fyz[p] = ZEO;
+    {
+        /* k0=ex3-1 面 */
+        for (int j0 = 0; j0 < ex2; ++j0)
+            for (int i0 = 0; i0 < ex1; ++i0) {
+                const size_t p = idx_ex(i0, j0, ex3-1, ex);
+                fxx[p]=ZEO; fyy[p]=ZEO; fzz[p]=ZEO;
+                fxy[p]=ZEO; fxz[p]=ZEO; fyz[p]=ZEO;
+            }
+        /* j0=ex2-1 面 */
+        for (int k0 = 0; k0 < ex3-1; ++k0)
+            for (int i0 = 0; i0 < ex1; ++i0) {
+                const size_t p = idx_ex(i0, ex2-1, k0, ex);
+                fxx[p]=ZEO; fyy[p]=ZEO; fzz[p]=ZEO;
+                fxy[p]=ZEO; fxz[p]=ZEO; fyz[p]=ZEO;
+            }
+        /* i0=ex1-1 面 */
+        for (int k0 = 0; k0 < ex3-1; ++k0)
+            for (int j0 = 0; j0 < ex2-1; ++j0) {
+                const size_t p = idx_ex(ex1-1, j0, k0, ex);
+                fxx[p]=ZEO; fyy[p]=ZEO; fzz[p]=ZEO;
+                fxy[p]=ZEO; fxz[p]=ZEO; fyz[p]=ZEO;
+            }
     }
 
     /*