index.html

<view class='select_box'>
    <view class='select' catchtap='selectTap'>
        <text class='select_text'>{{selectData[index]}}</text>
        <image class='select_img {{show&&"select_img_rotate"}}' src=''></image>         
    </view>
    <view
 
 class='option_box' style='height:{{show?(selectData.length>5?300:selectData.length*60):0}}rpx;'>
        <text class='option' style='{{index==selectData.length-1&&"border:0;"}}' wx:for='{{selectData}}' wx:key='this' data-index='{{index}}' catchtap='optionTap'>{{item}}</text>
    </view
 
>
</view>

index.wxss

page{
  background: #f3f7f7;
}
.select_box{
  background: #fff;
  width: 80%;
  margin: 30rpx auto;
  position: relative;
}
.select{
  box-sizing: border-box;
  width: 100%;
  height: 70rpx;
  border:1px solid #efefef;
  border-radius: 8rpx;
  display: flex;
  align-items
 
: center;
  padding: 0 20rpx;
}
.select_text{
  font-size: 30rpx;
  flex: 1;
}
.select_img{
  width: 40rpx;
  height: 40rpx;
  display: block;
  transition:transform 0.3s;
}
.select_img_rotate{
  transform:rotate(180deg); 
}
.option_box{
  position: absolute;
  top: 70rpx;
  width: 100%;
  border:1px solid #efefef;
  box-sizing: border-box;
  height: 0;
  overflow-y: auto;
  border-top: 0;
  background: #fff;
  transition: height 0.3s;
}
.option{
  display: block;
  line-height: 40rpx;
  font-size: 30rpx;
  border-bottom: 1px solid #efefef;
  padding: 10rpx;
}

index.js

Page({

  data: {
    show: false,//控制下拉列表的顯示隱藏，false隱藏、true顯示
    selectData: ['1', '2', '3', '4', '5', '6'],//下拉列表的資料
    index: 0//選擇的下拉列表下標
  },
  // 點選下拉顯示框
  selectTap() {
    this.setData({
      show: !this.data.show
    });
  },
  // 點選下拉列表
  optionTap(e) {
    let Index = e.currentTarget.dataset.index;//獲取點選的下拉列表的下標
    this.setData({
      index: Index,
      show: !this.data.show
    });
  },
  onLoad: function (options) {

  }

})