如下：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="description" content="">
<meta name="author" content="">
<link rel="icon" href="../../favicon.ico">
<title>Signin Template for Bootstrap</title>
<!-- Bootstrap core CSS -->
<link href="http://cdn.bootcss.com/bootstrap/3.3.2/css/bootstrap.min.css" rel="stylesheet">
<style>
.vertical-center {
position: absolute;
top: 40%;
left: 50%;
transform: translate(-50%, -50%);
}
</style>
</head>
<body>
<div class="container vertical-center" style="border:1px solid blue;">
<div class="col-lg-4 col-lg-offset-4 col-sm-6 col-sm-offset-3 col-xs-8 col-xs-offset-2" >Sign in</button>
</form>
</div>
</div> <!-- /container -->
</body>
</html>