function setSelectOption(objId, targetValue){
//objid：下拉列表框的ID；targetValue：当前所选值
 var obj = document.getElementById(objId);
 if(obj){
  var options = obj.options;
  if(options){
   var len = options.length;
   for(var i=0;i<len;i++){
    if(options[i].value == targetValue){
     options[i].defaultSelected = true;
     options[i].selected = true;
     return true;
    }
   }
  } else {
   alert('missing element(s)!');
  }
 } else {
  alert('missing element(s)!');
 }
}

<select>
  <option value="1" <%if(sesssionValue==1){%> selected="selected"<%}%>>1:b </option>
  <option value="2" <%if(sesssionValue==2){%> selected="selected"<%}%>>2:b </option>
  <option value="3" <%if(sesssionValue==3){%> selected="selected"<%}%>>3:c </option>
  <option value="4" <%if(sesssionValue==4){%> selected="selected"<%}%>>4:d </option>
</select>