非常简单就实现了，代码不用解释，很容易懂的   
   
//request data for centering
var windowWidth = document.documentElement.clientWidth;
var windowHeight = document.documentElement.clientHeight;
var popupHeight = $("#popupContact").height();
var popupWidth = $("#popupContact").width();
//centering
$("#popupContact").css({
    "position": "absolute",
    "top": windowHeight/2-popupHeight/2,
    "left": windowWidth/2-popupWidth/2
});