话不多说，先看效果图
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>上下左右居中</title>
<style type="text/css">
.box {
width: 200px;
height: 100px;
border: 1px solid red;
/* 上下左右居中，下面四节通用 */
position: fixed;
left: 50%;
top: 50%;
transform: translate(-50%, -50%);
}
</style>
</head>
<body>
<div class="box">
</div>
</body>
</html>