I've changed the weapons121.ini,and I try it but it doesn't work.(e.g. I changed the ammo of socom from 12 to 24 but in game it still appear as 12 shots)
What sgould I do then?

Well, I saw this line in the front page of soldat.pl:
"Added /loadwep command works with other files eg. /loadwep weapons121 (loads weapons121.ini settings)"

Change weapons.ini.
weapons121.ini is just a file with version 1.2.1 weapon settings.