Abstract: The study aims at the toxicological effect of ammonia nitrogen on Allogeneic crucian carp‘CAS Ⅲ’. The aquatic organism toxicity method was used under hydrostatic condition indoor. The acute toxicity test of ammonia nitrogen on juvenile of Allogynogenetic gibel carp‘CAS Ⅲ ’was conducted, and the changes of catalase (CAT), total superoxide dismutase (T-SOD) and malondialdehyde (MDA) in liver tissues were tested. The results showed that after ammonia nitrogen stress, the median lethal concentration (LC50) of total ammonia nitrogen was 276.51 mg/L at 24 h, 190.62 mg/L at 48 h and 148.62 mg/L at 96 h, and the LC50 of un-ionized ammonia was 2.52 mg/L at 24 h, 1.74 mg/L at 48 h, and 1.36 mg/L at 96 h. The safe concentration of total ammonia nitrogen and un-ionized ammonia was 14.87 mg/L and 0.14 mg/L, respectively, for juvenile. Under ammonia nitrogen stress, the content of MDA in the liver increased by 26.20%, 18.68% and 17.08% compare with the control (P<0.05), but CAT and T- SOD appeared to be activated at first and then inhibited, and antioxidase was induced more significantly at relatively higher concentration of ammonia nitrogen and long time exposure, and it weakened the antioxidant capacity of the body. The results reveal that ammonia tolerance of Allogynogenetic gibel carp‘CAS Ⅲ’is higher than that of Mylopharyngodon piceus, Ctenopharyngodon idellus, Hypophthalmichthys molitrix (Valenciennes, 1844), Cyprinus carpio Linnaeus, and other conventional freshwater fish. However, acute ammonia nitrogen stress damaged the liver antioxidant system and it is harmful to the health and growth of juvenile fish.