用window.location="xxxxxxxx";

用window.location="xxxxxxxx";试试看。
另外，按键以后要判断一下是不是回车。

<!doctype html>
<html>
    <head>
        <title>Enter Goto URL</title>
        <script type="text/javascript" src="jquery.js"></script>
        <script type="text/javascript">
            function isearch() {  
                var s="/search/";  
                var key=encodeURI(encodeURI($("#key").val()));  
                var city=$("#city").val();  
                var page=$("#pageid").val();  
                window.location=s+"s?k="+key+"&city="+city+"&page="+page;  
            }
            $(function() {
                $("#key").keyup(function(event){
                  if(event.keyCode == 13){
                    isearch();
                  }
                });
            });
        </script>
    </head>
    <body>
        <div>  
            <input id="city" type="hidden" value="zhengzhou"/>  
            <input id="pageid" type="hidden" value="1"/>  
            <input id="key" type="text"/>   
            <input type="button" id="btn" title="搜索" onclick="isearch()"/>  
        </div>  
    </body>
</html>

用window.location=&quot;xxxxxxxx&quot;;

用window.location="xxxxxxxx";