if object_id('aaa') is not null 
drop table aaa
go
create table aaa(
Id int identity(1,1),
No int null,
Name varchar(20))
go
create trigger aaa_ins
on aaa
after insert
as
   update aaa set No=i.Id
        from inserted i 
        where aaa.Id=i.Id
go
insert into aaa(Name) values('a');
select * from aaa;

------解决方案--------------------
try this,
SQL code

create table aaa
( Id int identity(1,1),
  No int default(isnull(scope_identity(),0)+1),
  Name varchar(20)
)

insert into aaa(Name) values('a')

select * from aaa

/*
Id          No          Name
----------- ----------- --------------------
1           1           a

(1 row(s) affected)
*/