html:

<div v-for = "(item,index) in items" :class = "active == index ? 'addclass' : '' " @click='dowm(index)'>{{item.name}}</div>

css:

css:.addclass{border-bottom: 3px solid #fb7299;color: #fb7299;}

js:

items :[{name ：'点我1',},{name:'点我2',},{name:'点我3',}],active : -1}dowm(index){//将点击的元素的索引赋值给变量this.active = index}