function getSelectionText(doc) {
    if (!doc) doc = document;
    var q;
    try {
        if (doc.parentWindow && doc.parentWindow.getSelection) q = doc.parentWindow.getSelection();
        else if (doc.getSelection) q = doc.getSelection();
        else if (doc.selection) q = doc.selection.createRange().text;
    } catch(e) {}

    if (!q) {
        var iframes = doc.getElementsByTagName("iframe");
        for (var i = 0; i < iframes.length; i++) {
            try {
                q = this.getSelectionText(iframes[i].contentWindow.document);
            } catch(e) {}
            if (q) return q;
        }
    }
    return q;
}

------解决方案--------------------

 <script> 
     function Test() {  
   
         try {  
             var selecter = window.getSelection();  
   
             if (selecter != null) {  
                 alert(selecter);  
             }  
         } catch (err) {  
             var selecter = document.selection.createRange();  
             var s = selecter.text;  
             if (s != null && s.trim() != "") {  
                 alert(s)  
             }  
        }  
     }  
   
    String.prototype.trim = function() {  
         return this.replace(/(^\s*)|(\s*$)/g, "");  
     }  
  </script>