The invention provides gas generant for air bag reducing the residue of sulfur as impurity having high quality and even grain diameter formed by BHT ammonium salt and the preparing method thereof. The preparing method of the gas generant for air bag is characterized in: adding with 1.05 to 1.2 times mol of sodium carbonate to react with 5,5'-cross-1H-tetrazole manganese salt under the existence of hydrosolvent, filtering the manganese as manganous carbonate, adding with 1.05 to 1.2 times mol of ammonium sulphate under the existence of hydrosolvent, heating the temperature to the range of 70 to 95 centigrade to ensure the determined temperature to react with the 5,5'-cross-1H-tetrazole sodium salt.