Objective To investigate the effects of dietary addition of Astragalus polysaccharides on the growth, feed coefficient, intestinal histology, and serum indices of Nile tilapia, providing a basis for the rational application of Astragalus polysaccharides in tilapia feed. Methods The basal diet was used as a control group, while the experimental group received an additional 0.5 g/kg of Astragalus polysaccharides in the basal feed, forming two isonitrogenous and isolipidic diets that were fed to Nile tilapia of similar initial body weight. After 8 weeks of feeding, growth performance and body indices, muscle routine components, digestive enzyme activity, and serum antioxidant capacity were measured, and intestinal tissue sections were observed. Results Compared to the control group, the treatment group with added astragalus polysaccharides showed an increase in final mass by 21.25 g, a weight gain rate improvement by 63.11 percentage points, and a feed conversion ratio reduction of 0.29. The muscle crude fat in the astragalus polysaccharide group significantly increased by 0.08 percentage points. The villus height of the intestines in the Astragalus polysaccharide group increased by 63.16 μm, and the thickness of the muscular layer increased by 4.57 μm, both significantly higher than those of the control group; the activity of amylase and lipase in the intestines significantly increased. After the ammonia nitrogen challenge test, the addition of astragalus polysaccharides to the diet significantly enhanced the intestinal superoxide dismutase (SOD) activity in tilapia, increasing by 6.37 U/L on the third day, and also increased the serum catalase (CAT) activity, although the difference was not significant. Conclusions The addition of Astragalus polysaccharides to the feed can improve the growth performance, intestinal digestion, morphology, and antioxidant capacity under ammonia nitrogen challenge of Nile tilapia.