SOURCE

<div class="box"> 
  <div class="Abox"> 我是刷黄飞鸿我是刷黄飞鸿我是刷黄飞鸿我是刷黄飞鸿 我是刷黄飞鸿 你钢结构 </div>
</div>

  *{
			padding:0;
			margin: 0;
		}
    html,body{
        width: 100%;
        height: 100%;
    }
		.box{
			background: red;
			width: 100%;
			height: 100%;
      position: relative;
		}
		.Abox{
      position: absolute; 
      /* top:0;
      left:0;
      right:0;
      bottom:0;
      margin:auto; */
      top:50%;
      left:50%;
      transform:translate(-50%,-50%);
			background: yellow;
			font-size: 20px; 
      width: 20%;
			height: 0;
			padding-bottom: 10%;    //20%*0.5
      overflow:auto;
		}
}