问题找到了，因为模板页中有一个div 的css属性为position:relative;现在我把问题简化成以下代码：
各位只看黑体部分即可，如果把position:absolute去掉，那么在ie6~ie8下都可以正常显示，如果把position:relative加上那么在ie6下遮罩层会遮盖所有层，不管z-index如何，我不知道原因，一点点试出来的，求教各位高人原因，以下代码直接复制即可执行。（注意要在ie6下做实验，因为ie8下是正常的）
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
function show_Win(div_Win, tr_Title, id) {
var iWidth = document.documentElement.clientWidth;
var iHeight = document.documentElement.clientHeight;
var bgObj = document.createElement("iframe");
bgObj.setAttribute("id", "div_Bg");
bgObj.style.position = "absolute";
bgObj.style.left = "0px";
bgObj.style.top = "0px";
bgObj.style.width = iWidth + "px";
bgObj.style.height = Math.max(document.body.clientHeight, iHeight) + "px";
bgObj.style.backgroundColor = "#000000";
bgObj.style.filter = "Alpha(Opacity = 60)";
bgObj.style.Opacity = 0.6;
bgObj.style.zIndex = "1";
document.body.appendChild(bgObj);
var msgObj = document.getElementById(div_Win);
msgObj.style.display = '';
msgObj.style.position = "absolute";
msgObj.style.top = (iHeight - msgObj.offsetHeight) / 2 + "px";
msgObj.style.left = (iWidth - msgObj.offsetWidth) / 2 + "px";
msgObj.style.backgroundColor = "#FFFFFF";
msgObj.style.border = "double 4px #808080";
msgObj.style.zIndex = "99999";
var moveX = 0;
var moveY = 0;
var moveTop = 0;
var moveLeft = 0;
var moveable = false;
var docMouseMoveEvent = document.onmousemove;
var docMouseUpEvent = document.onmouseup;
var titleBar = document.getElementById(tr_Title);
titleBar.style.cssText = "cursor:move;";
titleBar.onmousedown = function() {
var evt = getEvent();
moveable = true;
moveX = event.clientX;
moveY = evt.clientY;
moveTop = parseInt(msgObj.style.top);
moveLeft = parseInt(msgObj.style.left);
document.onmousemove = function() {
if (moveable) {
var evt = getEvent();
var x = moveLeft + evt.clientX - moveX;
var y = moveTop + evt.clientY - moveY;
if (x > 0 && (x + msgObj.offsetWidth < iWidth) && y > 0 && (y +
msgObj.offsetHeight < iHeight)) {
msgObj.style.left = x + "px";
msgObj.style.top = y + "px";
}
}
};
document.onmouseup = function() {
if (moveable) {
document.onmousemove = docMouseMoveEvent;
document.onmouseup = docMouseUpEvent;
moveable = false;
moveX = 0;
moveY = 0;
moveTop = 0;
moveLeft = 0;
}
};
}
}
function getEvent() {
return window.event || arguments.callee.caller.arguments[0];
}
function close_Win(div_Win) {
var div_Bg = document.getElementById("div_Bg");
var div_Add = document.getElementById(div_Win);
document.body.removeChild(div_Bg);
div_Add.style.display = 'none';
}
function StringToArray(str, substr) {
var arrTmp = new Array();
if (substr == "") {
arrTmp.push(str);
return arrTmp;
}
var i = 0, j = 0, k = str.length;
while (i < k) {
j = str.indexOf(substr, i);
if (j != -1) {
if (str.substring(i, j) != "") { arrTmp.push(str.substring(i, j)); }
i = j + 1;
} else {
if (str.substring(i, k) != "") { arrTmp.push(str.substring(i, k)); }
i = k;
}
}
return arrTmp;
}
</script>
</head>
<body>
<input type="button" id="btn" onclick="show_Win('add_Win','title_Bar')" value="弹出" />
<div style="position: relative; margin: 0px 0px 0px 14%; padding: 0px; border: solid 2px blue;
width: 500px; height: 600px;">
<div id="add_Win" style="display: none; position: absolute; z-index: 3; width: 300px;
height: 300px; border: solid 10px blue;">
<table>
<tr>
<td id="title_Bar">
标题
</td>
</tr>
<tr>
<td>
内容
</td>
</tr>
</table>
</div>
</div>
</body>
</html>
