1 .parent{
 2   width:500px;
 3   height:500px;
 4   background:green;
 5   position:relative;
 6 }
 7 .children{
 8   width:200px;
 9   height:200px;
10   background:blue;
11   position:absolute;
12   left:0;
13   top:0;
14   right:0;
15   bottom:0;
16   margin:auto;
17 }
18 <div class="parent">
19   我是parent
20   <div class="children">
21     我是children
22   </div>
23 </div>