.box {/*大盒子*/
400px;
height: 400px;
background: #Ccc;
position: relative;
}
.box1 {/* 小盒子 */
200px;
height: 100px;
background: red;
margin: auto;
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
}