div class=\'big2\'>
<div class=\'small2\'>box3</div>
</div>
.big2{
height:200px;
width:200px;
border:black solid 1px;
text-align:center;
}
.small2{
height:100px;
width:100px;
background-color:green;
margin:50px auto; //外面的div高度的一半
border:black solid 1px;
}
<div class=\'big\'>
<div class=\'small\'>box1</div>
</div>
style样式：
.big{
height:200px;
width:200px;
border:black solid 1px;
position:absolute;
left:150px;
}
.small{
height:100px;
width:100px;
background-color:green;
position:relative;
left:100px;
top:100px;
margin-top:-50px;
margin-left:-50px;
border:black solid 1px;
}