直接上代码

 

<!doctype html>
<html lang=”en”>
<head>
    <meta charset=”UTF-8”>
    <title>fixed</title>
</head>
<style>
div{
    position:fixed;
    margin:auto;
    left:0; right:0; top:0; bottom:0;
    width:200px; height:150px;
    background: red;
}
</style>
<body>
    <div></div>
</body>
</html>