方法一：.element { position: relative; top: 50%; transform: translateY(-50%);}
方法二：
HTML:
<body>
<div class="a1">
<div class="a2"></div>
</div>
</body>
css:
.a1{ 100%;height: 500px;display: table-cell;text-align: center;vertical-align: middle; }
.a2 { 50px; height:50px; background:green;display: table; }