在网上找了好多，实现方式大多是使用定时器
个人觉得定时器会造成一定程度上的假象
（如：定时器还没结束页面已经加载完毕，或定时器结束后页面还没加载完成）
本方法利用的是javascript，先网页面里面写入一个层，显示加载框，然后等页面结束后
在body的onload中写入事件，隐藏该对话框
方法（我的系统是多个页面，所以写了一个公用的类common.cs）：
在类里面加入如下函数（也可在每个代码后文件中写入）：
#region "页面加载中效果"
/// <summary>
/// 页面加载中效果
/// </summary>
public static void initJavascript()
{
HttpContext.Current.Response.Write(" <script language=JavaScript type=text/javascript>");
HttpContext.Current.Response.Write("var t_id = setInterval(animate,20);");
HttpContext.Current.Response.Write("var pos=0;var dir=2;var len=0;");
HttpContext.Current.Response.Write("function animate(){");
HttpContext.Current.Response.Write("var elem = document.getElementById('progress');");
HttpContext.Current.Response.Write("if(elem != null) {");
HttpContext.Current.Response.Write("if (pos==0) len += dir;");
HttpContext.Current.Response.Write("if (len>32 || pos>79) pos += dir;");
HttpContext.Current.Response.Write("if (pos>79) len -= dir;");
HttpContext.Current.Response.Write(" if (pos>79 && len==0) pos=0;");
HttpContext.Current.Response.Write("elem.style.left = pos;");
HttpContext.Current.Response.Write("elem.style.width = len;");
HttpContext.Current.Response.Write("}}");
HttpContext.Current.Response.Write("function remove_loading() {");
HttpContext.Current.Response.Write(" this.clearInterval(t_id);");
HttpContext.Current.Response.Write("var targelem = document.getElementById('loader_container');");
HttpContext.Current.Response.Write("targelem.style.display='none';");
HttpContext.Current.Response.Write("targelem.style.visibility='hidden';");
HttpContext.Current.Response.Write("}");
HttpContext.Current.Response.Write("</script>");
HttpContext.Current.Response.Write("<style>");
HttpContext.Current.Response.Write("#loader_container {text-align:center; position:absolute; top:40%; width:100%; left: 0;}");
HttpContext.Current.Response.Write("#loader {font-family:Tahoma,Helvetica, sans; font-size:11.5px; color:#000000;background-color:#FFFFFF; padding:10px 0 16px 0; margin:0 auto;display:block; width:130px; border:1px solid #5a667b; text-align:left;z-index:2;}");
HttpContext.Current.Response.Write("#progress {height:5px;font-size:1px; width:1px; position:relative; top:1px; left:0px;background-color:#8894a8;}");
HttpContext.Current.Response.Write("#loader_bg{background-color:#e4e7eb; position:relative; top:8px; left:8px;height:7px; width:113px; font-size:1px;}");
HttpContext.Current.Response.Write("</style>");
HttpContext.Current.Response.Write("<div id=loader_container>");
HttpContext.Current.Response.Write("<div id=loader>");
HttpContext.Current.Response.Write("<div align=center>页面正在加载中 ...</div>");
HttpContext.Current.Response.Write("<div id=loader_bg><div id=progress> </div></div>");
HttpContext.Current.Response.Write("</div></div>");
HttpContext.Current.Response.Flush();
}
#endregion
}
之后在需要调用的页面中在Page_Load()时间调用该方法：
Common.initJavascript();
并在每个页面body加载完毕后调用隐藏该div的事件
<body onload="remove_loading();">
完毕！