if (!ModelState.IsValid)

            {

                List<string> sb = new List<string>();

                //获取所有错误的Key

                List<string> Keys = ModelState.Keys.ToList();

                //获取每一个key对应的ModelStateDictionary

                foreach (var key in Keys)

                {

                    var errors = ModelState[key].Errors.ToList();

                    //将错误描述添加到sb中

                    foreach (var error in errors)

                    {

                        sb.Add(error.ErrorMessage);

                    }

                }

                return Json(sb);

            }

ModelState.AddModelError("UserName", "请输入用户名。");