[Required]
[Display(Name= "用户名")]        
[Remote("CheckUsn", "User", ErrorMessage="用户名已存在！")]
public string Usn { get; set; }

using System;
using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    public class UserController : Controller
    {
        public ActionResult CheckUsn(string Usn)
        {
            bool result = !Usn.Equals("admin", StringComparison.CurrentCultureIgnoreCase);
            return new JsonResult { Data = result, JsonRequestBehavior = JsonRequestBehavior.AllowGet };
        }
    }
}