給父元素加上after僞類：
.box:after {
content:'';
display: block;
clear:both;
}
例如：
<!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">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
.box{
width: 500px;
border: 1px solid deepskyblue;
}
.box>div{
width: 100px;
float: left;
height: 100px;
padding: 10px;
margin: 30px;
background-color: black;
}
.box:after{
content:'';
display: block;
clear:both;
}
</style>
</head>
<body>
<div class="box">
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
</div>
</body>
</html>