diff --git a/vd960Loop/utilities/at32f421_freertos_demo/src/TaskLoop.c b/vd960Loop/utilities/at32f421_freertos_demo/src/TaskLoop.c
index 537152c..29899b5 100644
--- a/vd960Loop/utilities/at32f421_freertos_demo/src/TaskLoop.c
+++ b/vd960Loop/utilities/at32f421_freertos_demo/src/TaskLoop.c
@@ -480,12 +480,19 @@ void vd1_task_per_channel(Loop154_Unit *unit)
 
         unit->loop_dlt_ORG = ((uint32_t)unit->loop_Origin * g_loop_sens_list.sens[unit->loop_SensLevel].sens_in) >> 16;
         {
+            /* V4B V4.20 同步: 双向对称保护 — 负variation(铁块磁导率)也冻结基线
+             * 原单边只挡 dev>+4×dlt, 负variation会把Origin污染抬高 → 离开时假进入 → 锁死不释放
+             * 修复: ① 对称窗口 [-4×dlt, +4×dlt] ② 负偏差只冻结永不超时更新 */
             int32_t dev = (int32_t)unit->loop_CAPVD - (int32_t)unit->loop_Origin;
-            if (dev < (int32_t)(unit->loop_dlt_ORG * 4)) {
+            int32_t freeze_band = (int32_t)(unit->loop_dlt_ORG * 4);
+            if (dev < freeze_band && dev > -freeze_band) {
+                /* 对称窗口内 → 正常基线跟踪 */
                 unit->loop_freeze_cnt = 0;
+                unit->loop_freeze_ref = 0;
                 update_moving_average(&unit->loop_ORG_SUM, &unit->loop_ORG_CNT,
                                       &unit->loop_Origin, unit->loop_CAPVD, WINDOW_ORIGIN);
-            } else {
+            } else if (dev < 0) {
+                /* 正variation (疑似车) → 冻结, 保留超时兜底更新 */
                 if (unit->loop_freeze_cnt == 0) {
                     unit->loop_freeze_ref = unit->loop_CAPVD;
                 } else {
@@ -509,6 +516,15 @@ void vd1_task_per_channel(Loop154_Unit *unit)
                     unit->loop_ORG_CNT = 0;
                     unit->loop_ORG_SUM = 0;
                 }
+            } else {
+                /* 负variation (CAPVD高于Origin, 特殊铁块磁导率主导) → 只冻结, 永不更新 Origin */
+                if (unit->loop_freeze_cnt == 0) {
+                    unit->loop_freeze_ref = unit->loop_CAPVD;
+                }
+                unit->loop_freeze_cnt++;
+                if (unit->loop_freeze_cnt > FREEZE_TIMEOUT) unit->loop_freeze_cnt = FREEZE_TIMEOUT;
+                unit->loop_ORG_CNT = 0;
+                unit->loop_ORG_SUM = 0;
             }
         }