1.已知盒子的寬高

.box{
   position: absolute;
    left:50%;
    top:50%;
    width: 100px;
    height: 100px;
    margin-left: -50px;
    marigin-top: -50px;
}

2.未知盒子的寬高

.box{
   position: absolute;
    left:50%;
    top:50%;
    transform : translate(-50%,-50%);