<a href="javascript:op(-1)">-</a><input value="1" type="text" id="txt" /><a href="javascript:op(1)">+</a>
<script type="text/javascript">
    function op(unit) {
        var txt = document.getElementById('txt'), v = txt.value;
        txt.value = parseInt(v, 10) + unit;
    }
</script>

------解决方案--------------------

JScript code

 <script src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript">
    function op(unit) {
 $("#txt").val(parseInt($("#txt").val())+unit); 
    }
</script>