Abstract: To develop a new type of aquatic feed additive for promoting growth and enhancing immune function, we cultured Epinephelus fuscoguttatus with body mass of (44.48±2.06) g for 56 d by adding fermented Astragalus membranaceus with proportions of 0% (Control group), 0.25%, 0.5%, 1%, 2% and 4% (L0, L1, L2, L3, L4 and L5 groups), and then conducted an ammonia nitrogen stress test. Taking growth, digestion and immune function as the indicators, we evaluated the physiological, biochemical and stress-resistant effects of fermented A. membranaceus on E. fuscoguttatus, and determined the optimal ratio. The results show that: 1) Compared with the control group, the addition of fermented A. membranaceus increased the weight gain rate (WGR), specific growth rate (SGR), and the activities of digestive enzymes in gastrointestinal tract significantly (P<0.05), but reduced the feed conversion ratio (FCR) of E. fuscoguttatus in L3 and L4 groups significantly (P<0.05). 2) Before the ammonia nitrogen stress, fermented A. membranaceus increased the liver antioxidant performance significantly (Except L5 group, P<0.05), but decreased the liver malondialdehyde (MDA) content and serum glutamic-oxalacetic transaminase (GOT) activity significantly (P<0.05). The serum glutamic-pyruvic transaminase (GPT) activity, glucose (GLU) concentration and serum triglyceride (TG) concentration in L3, L4 and L5 groups were significantly lower than those in the control group (P<0.05). 3) After the ammonia nitrogen stress, fermented A. membranaceus did not improve the antioxidant performance of liver, but reduced the MDA content in liver (Except L5 group), the activities of GOT and GPT as well as the GLU concentration in serum (Except L4 and L5 groups) significantly (P<0.05). In conclusion, the recommended feed additive proportion of fermented A. membranaceus is 1%–2%.