使用ajaxhandler--prototype

<script>
    var myGlobalHandlers = {
        onCreate: function(){
            Element.show('systemWorking');
        },

        onComplete: function() {
            if(Ajax.activeRequestCount == 0){
                Element.hide('systemWorking');
            }
        }
    };

    Ajax.Responders.register(myGlobalHandlers);
</script>

<div id='systemWorking'><img src='spinner.gif'>Loading</div>