<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<style>
.container{display: flex;align-items: center}
#a{height: 500px;width: 500px;background-color: royalblue;}
#b{height: 250px;width: 250px;background-color: darkblue;}
</style>
</head>
<body>
<div class="container">
<div id="a"></div>
<div id="b"></div>
</div>
</body>
</html>
如上所示三个div 内部两个div大小不一样 给外部div加上display: flex;align-items: center样式就可以使得较小的div垂直居中