SOURCE

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

.box {
  border:1px solid #aaa;
  width:400px;
  height: 400px;
  background-color:yellow;
  position: relative;
}
.son {
  height: 100px;
  width: 100px;
  border: 1px solid;
   background-color:green;
  /* x、y 轴 平移50% */
  position: absolute;   
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}