top属性是字符串。请尝试document.getElementById('division').style.top=x + 'px';
我尝试了下面的代码，它在IE和Firefox上运行良好。OnClick是IE的，AddEventListener是兼容的浏览器的。

<div id="clickMe" style="position: absolute; top: 10px;" onclick="myClick();">
    clickMe
</div>
<div id="division" style="position: absolute; top: 40px;">
    division
</div>
<script type="text/javascript">
    var clickMe = document.getElementById('clickMe');
    clickMe.addEventListener('click', myClick, 'false');

    function myClick(e) {
        var evt = e || window.event; //windows.event is for IE
        var x = evt.clientX;
        var y = evt.clientY;            
        document.getElementById('division').style.top = y + 'px';
        document.getElementById('division').style.left = x + 'px';
    }
</script>