Abstract: In this experiment,Bacillus with the degradation effect of ammonia nitrogen,including free ammonia (NH₃-N) and ammonium salt (NH⁺₄),was isolated and screened from a breeding outdoor pond of shrimp (Litopenaeus vannamei) under(28±1)℃.The single factor experiment was used to optimize the fermentation formula of the screened Bacillus and select the low cost and high yield fermentation medium formula.The results of strain isolation and screening,and ammonia nitrogen degradation experiment showed that:after preliminary isolation,five strains of Bacillus (DS1,DS2,DS3,DS4 and DS5) were obtained,and then one strain of Bacillus DS5 that could significantly degrade ammonia nitrogen in water was obtained through ammonia nitrogen degradation experiment.Within five days,the DS5 strain rapidly degraded the ammonia nitrogen in water,reaching the maximum degradation rate (36.41±0.07)% on the 4th day,and the ammonia nitrogen in the last 3 days water remained stable at about 36%.The strain DS5 was preliminarily identified as Bacillus subtilis by 16S rDNA sequence analysis as well as physiological and biochemical identification.Fermentation formula experimental results of strain DS5 showed that under 28℃,when carbon source adding 5‰ glucose,nitrogen source adding 10‰ soybean meal and inorganic salt adding 5‰ sodium chloride,the strain grew well,and the number of bacteria in culture medium was higher than others.To consider the cost and the amount of bacteria,5‰ glucose-10‰ soybean meal-5‰ sodium chloride was selected as the optimum fermentation medium for this strain.