Abstract: In order to study the effects of mushroom polysaccharides added into the diets on digestive enzymes, intestinal flora, muscle composition and tissue structure of loach under the ammonia-nitrogen stress, the healthy loach with initial weight of (8.9±0.3) g was selected and randomly divided into seven treatment groups. The control group was fed with basal diet, while the experimental group was fed with basal diet supplemented with 1.0 mg·g^-1 mushroom polysaccharides. The control groups and experimental groups were cultured in the corresponding water with the ammonia-nitrogen concentrations of 100 mg·L^-1, 200 mg·L^-1, and 300 mg·L^-1, respectively. In addition, the group without the ammonia-nitrogen stress was cultured in the water with the ammonia-nitrogen concentration of 0 mg·L^-1 and fed with basal diet. Each treatment was repeated for 3 times with 60 fish in each replicate. The experiment lasted for 35 days, and the samples were taken on the 7th, 14th, 21st, 28th and 35th day, respectively. The results showed that: the activities of protease, amylase and lipase in the digestive tract of loach in each experimental group were higher than those in the control group (P<0.05). On the 28th day, the activities of protease, amylase and lipase in the digestive tract of loach in the experimental group with 100 mg·L^-1 ammonia nitrogen stress were the highest, which were (1 953.8±39.31) U·mL^-1, (21.5±3.37) U·mL^-1 and (9.64±0.32) U·mL^-1, respectively. On the 35th day, the number of Lactobacillus and Bacillus in the digestive tract of loach in each experimental group was significantly higher than that in the control group (P<0.05), while the number of Escherichia coli was significantly lower than that in the control group (P<0.05). On the 35th day, the muscle moisture content of loach in each experimental group was significantly lower than that in the control group (P<0.05), while the protein content and fat content were significantly higher than those in the control group (P<0.05). Under the ammonia-nitrogen stress of 100 mg·L^-1, the tissue structure of hepatopancreas and gills of loach in the experimental group was relatively complete, and the cell structure was normal, while the tissue cell in the control group was necrotic, the tissue structure collapsed and the tissue cell function was impaired. The comprehensive analysis showed that mushroom polysaccharides could improve the activities of protease, amylase and lipase in the digestive tract of loach under the ammonia-nitrogen stress, increase the number of Lactobacillus and Bacillus in the digestive tract, reduce the number of Escherichia coli, increase the contents of protein and fat in muscle, reduce the water content, and improve the ability of resisting the ammonia-nitrogen stress in the gills and hepatopancreas of loach, which was beneficial to the health of fish.