分3步骤：
1 在页面下面加载JS：
.......
</html>
<script type="text/javascript">
mydiv.style.visibility = 'hidden';
</script>
2.在.cs文件 LOAD事件里：
DataLoad.Waiting();
3.静态类（单独一个文件）：
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Threading;
/// <summary>
/// PageLoad 的摘要说明
/// </summary>
public static class DataLoad
{
public static void Waiting()
{
System.Web.HttpContext.Current.Response.Write("<div ID=/"mydiv/" style=/"left: 200px; width: 150px; z-index: 10000; position: absolute; top: 120px; height: 25px/">正在装载页面......</div>");
System.Web.HttpContext.Current.Response.Flush();
}
}