$('#firstfactor').combobox({
url: '@Url.Action("GetMultiAirFactor_Day_New", "UtilManage")'
});
没有定义 valueField和textfield，修改如下
$('#firstfactor').combobox({
url: '@Url.Action("GetMultiAirFactor_Day_New", "UtilManage")',
valueField: 'id', textfield: 'text'
});