Abstract: The aim of this study was to obtain the aquatic probiotics with high-efficiency ammonia nitrogen-degrading ability and favorable probiotic properties. The ammonia nitrogen-degrading characteristics, safety (antibiotic susceptibility and hemolytic activity), and enzyme-producing capacity of the Acinetobacter johnsonii strain FJAT 49979 that was isolated from the aquaculture wastewater were systematically evaluated. Moreover, the fermentation medium for this strain was also optimized by using the viable cell count and ammonia nitrogen-degrading rate as the indicators. The results showed that the degradation rates of ammonia nitrogen by the strain FJAT-49979 were 98.92%, 76.11%, and 43.38% within 12 hours at the concentrations of 15, 35, and 70 mg·L⁻¹, respectively, indicating that it had the characteristics of rapid degradation of ammonia nitrogen and tolerance to high concentration of ammonia nitrogen. Furthermore, the strain FJAT-49979 was highly sensitive to nine commonly used antibiotics and non-hemolytic activity, and had the ability to produce mannanase. After the optimization, the optimal fermentation medium for the strain FJAT 49979 was determined as follows: peptone 7.5 g, NaCl 10 g, KH₂PO₄ 1.3 g, K₂HPO₄·3H₂O 2.8 g, and Juncao herb juice 1000 mL, pH 7.0. In conclusion, the strain FJAT 49979 exhibited high efficiency of ammonia nitrogen degradation ability, favorable safety profiles and probiotic potential, and was easy to ferment and produce. It had good application prospects in water quality regulation and health management in aquaculture.