diff --git a/backend/app/services/line_service.py b/backend/app/services/line_service.py
index b0ef04f..e9ad8e0 100644
--- a/backend/app/services/line_service.py
+++ b/backend/app/services/line_service.py
@@ -66,13 +66,27 @@ async def _saddle_plan(db: AsyncSession):
 
 
 async def ensure_online(db: AsyncSession):
-    """保证恰好一条 online（队首，最早录入的 ready）。鞍座上的计划不参与队列。"""
+    """单卷在产：有卷在上卷鞍座/生产中时不保留在线队首；否则把最早 ready 置为唯一在线。"""
     res = await db.execute(
         select(ProductionPlan).where(ProductionPlan.status == "online", ProductionPlan.on_saddle != 1)
     )
     online = list(res.scalars())
+
+    # 是否已有在产/在鞍座的卷
+    res2 = await db.execute(
+        select(ProductionPlan).where(
+            (ProductionPlan.status == "producing") | (ProductionPlan.on_saddle == 1)
+        )
+    )
+    active = res2.scalars().first() is not None
+
+    if active:
+        # 正在生产时，不再保留「在线」队首，全部回退 ready，等当前卷完成
+        for p in online:
+            p.status = "ready"
+        return
+
     if len(online) > 1:
-        # 仅保留最早的一条为 online，其余回退 ready
         online.sort(key=lambda p: (p.plan_date or datetime.max, p.id))
         for p in online[1:]:
             p.status = "ready"