.cover {
position:fixed; top: 0px; right:0px; bottom:0px;filter: alpha(opacity=60); background-color: #777;
z-index: 1002; left: 0px; display:none;
opacity:0.5; -moz-opacity:0.5;
}

body>

　　div class="container" style="height:2000px;">
　　　　div style="height:1000px;">/div>
　　　　a href="javascript:;" onclick="showMask()" >點我顯示遮罩層/a>br />
　　/div>
　　div id="cover" class="cover">/div>
/body>

function showMask(){
$('body').css("overflow","hidden")
$("#cover").show();
}