: The 3×2 two factors random animal experiment was conducted for 56 days to evaluate the effects of dietary fat and protein on digestive enzymes activities in alimentary canal and liver,nonspecific immunity of Atlantic salmon and the water environment reared at recirculation aquaculture systems(RAS).The trial fish(initial weight 650±45.5 g)were randomly allocated into 6 triplicated treatments and fed six kinds of diets in a factorial design containing three fat levels(18%,21%,24%,labeled as F18,F21,F24)and two protein levels(38%,48%,labeled as P38,P48).Results show that:(1)the stomachal,intestinal and hepatic lipase activities in medium fat level were significantly higher than that in low fat level by 11.52%(P>0.05),14.63%(P<0.05),4.31%(P>0.05)respectively;trypsin showed much higher activity than pepsin in gut and liver,and activities of trypsin in gut and liver were enhanced at high protein level by 8.23%(P>0.05)and 8.39%(P<0.05);(2)diet low fat and high protein levels could be good to promote the intestinal canal AKP activity of Atlantic salmon,intestinal AKP activity was significantly improved at low fat level by 18.61%,31.70%(P<0.05)than that at high and medium fat levels respectively,and was also enhanced by high protein level by 13.69%(P<0.01).(3)Atlantic salmon had a highest serum SOD activity at medium fat level and a lowest MDA activity at high fat level;high protein level was beneficial to increase the serum LZM activity and C3 concentration by 9.49%(P>0.05)and 5.93%(P<0.05).(4)diet low protein level can significantly reduce the content of ammonia nitrogen and nitrate in water by 61.70%(P<0.05)and 28.36%(P<0.01),diet high fat level could reduce water ammonia nitrogen content by 10.00%(P>0.05)and 8.20%(P>0.05)than medium and high level.This research revealed that appropriately increasing diet fat level could benefit the lipase activity,increasing diet protein level could benefit immunity,and the combination of low protein and high fat could effectively reduce the ammonia nitrogen excretion of Atlantic salmon.