Wenbin Zhang,
Since you're using SQL Server 2005, the best way to get this type of
statistical info is from the word list in the FT Catalog via the CIDump.exe
utility (see related thread subject: cidump documentation?) and import the
output back into a SQL Server table.

If you need to use a T-SQL to determine the word count of a SQL FTS query,
instead of just using a count(*), it is faster to use a stored proc, for
example:

create proc FTS_t1 (@SearchWord varchar(7800))
as
select c1 from t1 where contains(c2, @SearchWord)
go
create proc FTSCount_t1 (@SearchWord varchar(7800))
as
set nocount on
create table #FTPrimaryKey(t1_UPK int)
insert into #FTPrimaryKey exec FTS_t1 @SearchWord
select count(*) from #FTPrimaryKey
set nocount off
go
-- example of use:
exec FTSCount_t1 '"computer"'

Hope that helps!
John