//方法一
#box{
width:120px;
height:160px;
border:1px solid red;
position:absolute;
top:50%;
left:50%;
margin-left:-60px;
margin-top:-80px;
}
//方法二
#box{
width:120px;
height:160px;
border:1px solid red;
position:absolute;
top:50%;
left:50%;
transform: translate(-50%,-50%);
}