1.el-tree设置 node-key，current-node-key，default-expanded-keys，highlight-current：

<el-tree 
ref="taskTree" 
:data="ptreeData"
node-key="key"
:current-node-key="currentKey" 
:default-expanded-keys="defaultExpanded" 
highlight-current>
</el-tree>

2.赋值数据

 nextTick(() => {
            currentKey.value=firstNode.key;
            taskTree.value.setCurrentKey(firstNode.key);
            defaultExpanded.value=nodeIds;
 })

注意：使用了nextTick。