方法一(宽高不固定)

.box{
    width: 600px;
	height: 400px;
	position: absolute;
	left: 50%;
	top: 50%;   
	transform: translate(-50%, -50%);
}

方法二(宽高固定)

.box{
    width: 600px;
	height: 400px;
	position: absolute;
	left: 50%;
	top: 50%;   
	margin-top:-200px;
	margin-left:-300px;
}

方法三（不定宽高）

.box{
   position: absolute;
	left: 0;
	top: 0;   
	right:0;
	bottom:0;
	margin:auto;
}

方法四(不定宽高)

.box{
   display:flex;
   justify-content: center;
   align-items: center;
}

方法五(不定宽高)

.box{
  display: table-cell;
  vertical-align: middle;
  text-align: center;
}
//子节点
.box-item{
   display:inline-block;
}