HTML

<el-tree
  :data="data"
  show-checkbox
  node-key="id"
  :default-expanded-keys="defaultExpandedKeys"
  @node-expand="handleNodeExpand"
  @node-collapse="handleNodeCollapse">
</el-tree>

JS

<script>
export default {
  data() {
    return {
      // 树集合
      data: [
        {
          id: 1,
          label: '一级 2',
          children: [{
            id: 3,
            label: '二级 2-1',
            children: [{
              id: 4,
              label: '三级 3-1-1'
            }, {
              id: 5,
              label: '三级 3-1-2',
              disabled: true
            }]
          }
      ],
      // 默认展开的节点的 key 的数组
      defaultExpandedKeys: []
    }
  },
  methods: {
    // 当节点展开时记录节点id
    handleNodeExpand(data, node) {
      this.defaultExpandedKeys.push(data.id);
    },
    // 当节点收缩时删除节点id
    handleNodeCollapse(data, node){
      this.defaultExpandedKeys = this.defaultExpandedKeys.filter(item => item !== data.id)
    }
}
</script>