#brg
{
 width: 100%;
 height: 100%;
 background: #333;
 position: absolute;
 top: 0;
 left: 0;
 filter: alpha(opacity=60);
 -moz-opacity: 0.6;
 opacity: 0.6;
 position: absolute;
 top: 0;
 left: 0;
 display: none;
}
#showdiv
{
 width: 100%;
 height: auto;
 position: absolute;
 left: 300px;
 top: 150px;
 z-index: 330;
 display: none;
}
#testdiv
{
 width: 800px;
 height: auto;
 margin: 0 0;
 border: 1px solid #4d4d4d;
 background: #f2f2f2;
}
#close
{
 width: 200px;
 height: 27px;
 line-height: 27px;
 font-size: 14px;
 font-weight: bold;
 border: 1px solid #4d4d4d;
 text-align: center;
 cursor: pointer;
 margin: 0 auto;
 background: #333;
 color: #fff;
}

 <div id="brg">
    </div>
    <div id="showdiv">
        <div id="close">
            關閉
        </div>
        <div id="testdiv">

  要顯示的內容

        </div>
    </div>

 $(document).ready(function() {
            var bheight = document.body.clientHeight;
            $("#btnAdd").click(function() {
                $("#brg").css("display", "block");
                $("#showdiv").css("display", "block");
                $("#brg").css("height", document.body.scrollHeight);
                $("#showdiv").css("top", document.body.scrollTop + 100);
            });
            $("#close").click(function() {
                $("#brg").css("display", "none");
                $("#showdiv").css("display", "none");
            });
        });