jquery的ajax调用后台服务方法实现下拉框联动时，json解析有关问题

jquery的ajax调用后台服务方法实现下拉框联动时，json解析问题？
我在页面中通过ajax调用后台方法服务，实现联动！ajax方法如下：

$.ajax({

                        type: "post",

                        async: false,

                        contentType: "application/json; charset=utf-8",

                        data: "{id:'" + id + "'}",

                        url: "AllAssessResults.aspx/GetObjListByDeptId",    //后台webservice里的方法名称

                        dataType: "json",

                        success: function (data) {

                            alert(data);

                            var optionstring = "";

                            var dataObj = $.parseJSON(data);

                            //var dataObj = eval("(" + data + ")");


                            for (var i = 0; i < dataObj.length; i++) {

                                optionstring += "<option value=\"" + dataObj[i].usercode + "\" >" + dataObj[i].username + "</option>";

                            }


                            $("#selectObj").html(optionstring);


                        },

                        error: function (msg) {

                            alert("出错了！");

                        }

                    });