<div id="mask" class="mask"></div>

/***********-  遮罩层  -***********/

#mask{

            position: absolute;

            top: 0px;

            left:0px;

            background:#333;

            opacity:0.5;

            /* older safari/Chrome browsers */

           -webkit-opacity: 0.5;

           /* Netscape and Older than Firefox 0.9 */

           -moz-opacity: 0.5;

           /* Safari 1.x (pre WebKit!) 老式khtml内核的Safari浏览器*/

                 -khtml-opacity: 0.5;

             /* IE 4-9 */

            filter:alpha(opacity=50);

            /*This works in IE 8 & 9 too*/

            -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";

            z-index:;

            display:none;

        }

 function showMask(){

   $("#mask").css("height",$(document).height());

   $("#mask").css("width",$(document).width());

   $("#mask").show();

 }