The invention provides a method for preparing synthetic ammonia by using smeltery exhaust gas, which employs smltery exhaust gas as raw material to prepare hydrogen and nitrogen, comprising preparing hydrogen by transforming coke-oven gas and steam and preparing nitrogen by using side gas produces in the oxygen producing process by oxygen generator in steel-smelting course. By preparing hydrogen, nitrogen and steam raw gas needed for preparing synthetic ammonia by using the waste resource in the smeltery production process, the ammonia production cost is reduced, energy is saved, the environment aroud steel plant and ecological environment is protected, and the economical benefit and social benefit is outstanding.