一、默认展开 :default-openeds="openeds"

二、默认展开第一个子节点 default-active="1-1"

三、默认展开收起一个unique-opened="true"

<template>
  <div>
     <el-menu active-text-color="#66f2ff" background-color="transparent" :default-openeds="openeds" default-active="1-1" unique-opened="true">
      <el-sub-menu :index="item.phaseIndex" v-for="item in treeData" :key="item.id">
        <template #title>
          <span>{{item.name}}</span>
        </template>
        <el-menu-item-group v-for="item1 in item.children" :key="item1.id">
          <el-menu-item :index="item.phaseIndex +'-'+item1.linkIndex" @click="menuItem(item1.id)"></el-menu-item>
        </el-menu-item-group>
      </el-sub-menu>
    </el-menu>
  </div>
</template>
 
<script setup>
import { reactive, toRefs, ref, onBeforeMount, onMounted } from "vue";
import $http from "@/requset.js"
 
function UseVal() {
  const openeds =ref([1])
  const treeData = ref([])
  const UseAssentment = async () => {
    const {data,code} = await $htttp.get('/user/userinfo')
    treeData.value = data;
  };
  return { openeds , UseAssentment };
}
const { openeds , UseAssentment } = UseVal();
</script>