代码

 <template>
  <el-cascader
    v-model="select"
    :props="props"
    :placeholder="placeholder"
    :size="size"
    @change="handleChangeValue"
    ref="mCascader"
  ></el-cascader>
 </template>


....
<script>
 onOpen() {
    (this.$refs.mCascader as any).$el.click();
  }
</script>