The invention discloses an SCR denitrification process and a denitrification device of an ammonia injection grid adopting non-unidiameter injection orifices. Through the ammonia injection grid adopting the non-unidiameter injection orifices, the diameter size of the ammonia injection orifices is gradually reduced along with the flow direction of ammonia gas so that the quantity of ammonia gas injected by the ammonia injection orifices keeps same in the process of ammonia injection, therefore, the mixed length of ammonia gas and flue gas can not be affected, and the denitrification effect is ensured.