SOURCE

<div class="box">
    <div class="son"></div>
</div>

.box {
  border:1px solid #aaa;
  width:400px;
  height: 400px;
  background:green;
  position: relative;
}
.son {
  height: 100px;
  width: 100px;
  border: 1px solid;
  background:yellow;
  /* 代码如下 */
  position: absolute;    
  top: 0; bottom: 0; left: 0; right: 0; 
  margin: auto;
}