<div class="flex_wrap">
<div class="flex_item" style="height: 20px">1/div>
<div class="flex_item" style="height: 30px">2/div>
<div class="flex_item" style="height: 40px">3/div>
</div>.flex_wrap {
display: flex;
flex-direction: row; //主轴水平方向
flex-wrap: wrap;
justify-content: center; //主轴上（水平）的居中
align-items: center; //交叉轴（垂直）的居中
border: solid 1px blue;
}
.flex_item {
display: block;
background-color: red;
}效果如下图：
