*{ margin:0; padding:0;}
.bg{ width:500px; height:500px; position:relative; outline:1px solid #333; margin:100px auto;}
.box{ background:#000; position:absolute;margin:auto ;top:0; bottom:0; left:0; right:0;color:#FFF;width:110px; height:50px;}/*你可以改一下宽高试试*/