方式1：flex布局,display:flex;align-items:center

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <title></title>
    <style type="text/css">
        *{padding:0;margin:0;}
        .box{width:100px;height:100px;border:1px solid #ddd;display:flex;align-items:center;}
        .inner{width:40px;height:40px;background:red;}
    </style>
</head>
<body>
<div class="box">
   <div class="inner"></div>
</div>
</body>
</html>

效果如下：

 

 方式2：position+margin

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <title></title>
    <style type="text/css">
        *{padding:0;margin:0;}
        .box{width:100px;height:100px;border:1px solid #ddd;position:relative;}
        .inner{width:40px;height:40px;background:red;position:absolute;top:50%;margin-top:-20px;}
    </style>
</head>
<body>
<div class="box">
   <div class="inner"></div>
</div>
</body>
</html>

效果图：与上面一样

方式3：如果不知道具体高度，通过position+transform

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <title></title>
    <style type="text/css">
        *{padding:0;margin:0;}
        .box{width:100px;height:100px;border:1px solid #ddd;position:relative;}
        .inner{background:red;position:absolute;top:50%;transform:translateY(-50%);}
    </style>
</head>
<body>
<div class="box">
   <div class="inner">545454</div>
</div>
</body>
</html>

效果图：

方式4：display:table-cell+vertical-align:middle

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <title></title>
    <style type="text/css">
        *{padding:0;margin:0;}
        .box{width:100px;height:100px;border:1px solid #ddd;display:table-cell;vertical-align:middle;}
        .inner{background:red;}
    </style>
</head>
<body>
<div class="box">
   <div class="inner">545454</div>
</div>
</body>
</html>

 效果图：

 

方式5：box布局，box-orient控制子元素是水平分布还是垂直分布，box-align让子元素垂直居中,box-pack让子元素水平居中

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <title></title>
    <style type="text/css">
        *{padding:0;margin:0;}
        .box{width:100px;height:100px;border:1px solid #ddd;display:-webkit-box;-webkit-box-orient:vertical;-webkit-box-pack:center;-webkit-box-align:center;}
        .inner{background:red;height:20px;}
    </style>
</head>
<body>
<div class="box">
   <div class="inner">54</div>
   <div class="inner">33</div>
</div>
</body>
</html>

效果图：