只需要在父元素中添加 display: table;
子元素中添加 display: table-cell;vertical-align: middle;
上代码：
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title></title>
</head>
<style type="text/css">
* {
margin: 0;
padding: 0;
}
.box {
height: 200px;
100%;
border: 1px solid beige;
background-color: beige;
}
.box_in {
height: 40%;
100%;
text-align: center;
background-color: salmon;
display: table;
margin-top: 20px;
}
.box_in span {
display: table-cell;
vertical-align: middle;
}
</style>
<body>
<div class="box">
<div class="box_in">
<span>内容文字</span>
</div>
</div>
</body>
</html>