.taskSort {
    width: 100%;
    justify-content: space-around;
}
.taskSort span:first-child{
  width: 4rem;
}

.zkicon {
    margin-left: 0.5rem;
    width: 0;
    height: 0;
    border-left: 0.45rem solid transparent;
    border-right: 0.45rem solid transparent;
    border-top: 0.6rem solid #666;
}

.sortSel {
    position: relative;
    height: 2rem;
}

.sortSel ul {
    position: absolute;
    width: 6rem;
    top: 2rem;
    left: 0;
    z-index: 9;
    background-color: #f6f6f6;
    box-shadow: 0 0 4px rgba(0, 0, 0, 0.1);
}

.sortSel ul li{
  width: 100%;
  height: 2rem;
  padding-left: 0.5rem;
  border-radius: 7px;
}

      <div class="flex_row taskSort">
                <div v-for="(item,index) in selectList" class="sortSel">
                    <div class="" @click="openSelect(index)">
                        <span>{{item.title}}</span><span class="zkicon"></span>
                    </div>
                    <ul class="" v-show="selectShow[index]">
                        <li v-for="(item2,index2) in item.list" @click="selectItem(index,index2)">{{item2.value}}</li>
                    </ul>
                </div>
            </div>

            var vue = new Vue({
                el: "#app",
                data: {
                    selectList: [{
                        list: [],
                        title: "默认排序"
                    }, {
                        list: [],
                        title: "最新发布"
                    }, {
                        list: [],
                        title: "佣金最高"
                    }],
                    selectShow: [],
                },
                created: function() {
                    this.initSelect();
                },
                methods: {
                    initSelect: function() {
                        for (var i = 0; i < this.selectList.length; i += 1) {
                            this.selectShow[i] = false
                            for (var j = 0; j < 3; j += 1) {
                                this.selectList[i].list[j] = {}
                                this.selectList[i].list[j].name = "测试"
                                this.selectList[i].list[j].value = j
                            }
                        }
                    },
                    selectItem: function(i, j) {
                        this.selectList[i].title = this.selectList[i].list[j].value
                        this.openSelect(i);
                    },
                    openSelect: function(i) {
                            if (this.selectShow[i]) {
                                for (var i = 0; i < this.selectShow.length; i += 1) {
                                    this.$set(this.selectShow, i, false)
                                }
                            } else {
                                this.$set(this.selectShow, i, true)
                            }
                    },
                }
            })