<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
button { margin: 0 auto; display: block; }
ul { overflow: hidden; margin: 3% 0 0 32%; }
li { 100px; height: 100px; margin-right: 50px; background-color: #000; list-style: none; float: left;}
</style>
<script src="http://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
</head>
<body>
<button>点击循环改变颜色</button>
<ul>
<li></li>
<li></li>
<li></li>
</ul>
<script type="text/javascript">
$(document).ready(function(){
$("button").on("click", function(){
var num = $("li").length;
for (var i = 0; i < num; i++) {
$("li").eq(i).css("background-color", "red");
};
})
})
</script>
</body>
</html>