<input onclick="number('jian')" type="button" value="-" /> 
<span id="ss">1</span> 
<input onclick="number('jia')" type="button" value="+" /></p>

<script>
 function number(item) {
  var a = $("#ss").text();
  if (item == "jia") {
     a++;
  } else {
 　　 if (a != 1) {
        a--;
     }
  }
  $("#ss").text(a);
}
</script>