效果如下：
all code:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>jquery页面loading提示用户等待效果</title>
<style>
.mloadingDiv{
position:absolute;
left:0;
width:100%;
top:0;
background:#f3f8ff;
opacity:0.8;
filter:alpha(opacity=90);
z-index:10000;
}
.loadingDivChild{
position: absolute;
cursor: wait;
width: auto;
height:57px;
line-height:57px;
padding-left: 50px;
padding-right: 5px;
background: #fff url(images/gif.gif) no-repeat scroll 18px 20px;
border: 2px solid #95B8E7;
color: #696969;
font-family:\'Microsoft YaHei\';
}
</style>
</head>
<body>
<div class="mloadingDiv">
<div class="loadingDivChild">
正在从数据库请求数据，请您耐心等候.............
</div>
</div>
</body>
</html>
<script src="js/jquery-3.2.1.js"></script>
<script>
// 定义mloadingDiv的宽高及其子元素
$(".mloadingDiv").height(window.innerHeight);
var _Top = window.innerHeight > 61 ? (window.innerHeight - 61) / 2 : 0,
_Left = window.innerWidth > 215 ? (window.innerWidth - 215) / 2 : 0;
$(".loadingDivChild").css({
"left": _Left,
"top": _Top
});
</script>