<!DOCTYPE html>
<html lang="en">
<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>Document</title>
<style type="text/css">
body{height:1800px;}
#btn
{
position:fixed;
right:50px;
bottom:50px;
width:70px;
height:20px;
background-color: aquamarine;
color:white;
font-family: "宋体";
text-align:center;
font-weight:bold;
display:none;
cursor: pointer;
}
</style>
<script type="text/javascript" src="https://cdn.bootcss.com/jquery/2.1.1/jquery.min.js"></script>
<script type="text/javascript">
$(function(){
$(window).scroll(function(){
if($(this).scrollTop()>300){
$("#btn").css({"display":"inline-block"});
}else{
$("#btn").css({"display":"none"});
}
})
$("#btn").click(function(){
$("html,body").scrollTop(0);
})
})
</script>
</head>
<body>
<div id="btn">回到顶部</div>
</body>
</html>
注：按钮效果是通过div效果来实现的，在点击之后，响应点击事件回到顶部，即将scrollTop的值设置为0。