diff --git a/packages/reactivity/src/baseHandlers.ts b/packages/reactivity/src/baseHandlers.ts
index 43b6df903bd..4615aaeab1c 100644
--- a/packages/reactivity/src/baseHandlers.ts
+++ b/packages/reactivity/src/baseHandlers.ts
@@ -101,9 +101,9 @@ function createSetter(isReadonly = false, shallow = false) {
         }
       } else {
         if (!hadKey) {
-          trigger(target, TriggerOpTypes.ADD, key)
+          trigger(target, TriggerOpTypes.ADD, key, { newValue: value })
         } else if (hasChanged(value, oldValue)) {
-          trigger(target, TriggerOpTypes.SET, key)
+          trigger(target, TriggerOpTypes.SET, key, { newValue: value })
         }
       }
     }
diff --git a/packages/reactivity/src/effect.ts b/packages/reactivity/src/effect.ts
index 03326426d81..0cfa2202e4b 100644
--- a/packages/reactivity/src/effect.ts
+++ b/packages/reactivity/src/effect.ts
@@ -174,12 +174,20 @@ export function trigger(
   }
   const effects = new Set<ReactiveEffect>()
   const computedRunners = new Set<ReactiveEffect>()
-  if (type === TriggerOpTypes.CLEAR || (key === 'length' && isArray(target))) {
+  if (type === TriggerOpTypes.CLEAR) {
     // collection being cleared or Array length mutation
     // trigger all effects for target
-    depsMap.forEach(dep => {
+    depsMap.forEach((dep, i) => {
       addRunners(effects, computedRunners, dep)
     })
+  } else if (key === 'length' && isArray(target)) {
+    const newValue = extraInfo.newValue
+    const startIndex: number = (newValue as number) || 0
+    depsMap.forEach((dep, i) => {
+      if (i >= startIndex || i === 'length') {
+        addRunners(effects, computedRunners, dep)
+      }
+    })
   } else {
     // schedule runs for SET | ADD | DELETE
     if (key !== void 0) {