The interactive effects of ambient salinity decrement on liver antioxidant index and plasma lysozyme activity of Epinephelus Moara were investigated by rearing fish at salinity of 27,21,15 and 9 for 7 days. We tested activity of superoxide dismutase (SOD),catalase (CAT),glutathione peroxidase (GSH-Px), total antioxidant capacity (T-AOC), malondialdehyde (MDA)content, and plasma lysozyme activity at time periods of 0,1, 2, 3and 7 d respectively.The results showed that the liver SOD as well as CAT activities first dropped in 1,2 d (P < 0.05) and peaked in 3 d of Group 21, then dropped to normal in 7 d. The T-AOC activity was consistent with the SOD and CAT activities, while the MDA content was contrary. On the whole, plasma lysozyme activity dropped (P < 0.05) with the decrease of salinity and Group 21 peaked in 1 d. To sum up, the results indicate that ambient salinity decrement can effectively stimulate and inhibit the SOD and CAT activity in liver of Epinephelus Moara at initial stage, but it will recover then. T-AOC activity and MDA content will also correspondingly change. Plasma lysozyme activity first increased and then dropped with the elapse of treatment time,and thus it plays an important role in immune system.