Abstract: The global fishmeal is lacking, and the search for fishmeal substitutes has become a hot issue in the research of aquatic animal nutrition, which is of great practical significance. Soybean meal has a high protein content, a stable and low supply, and is considered as a good fish meal substitute, however, because it contains many anti-nutritional factors, including protease inhibitors, lectins, phytic acid, saponin, phytoestrogens, anti-vitamins and allergens, it will inhibit the digestion and absorption of nutrients to a certain extent. In addition, studies have shown that the replacement of fish meal with soybean meal can induce disorders of the intestinal microbiota of fish, and excessive replacement can also induce adverse reactions such as enteritis, oxidative stress and immune dysfunction in fish. However, soybean meal can reduce most anti-nutritional factors after microbial fermentation, and degrade macromolecular proteins into small peptides and water-soluble compounds, thereby increasing its nutritional value and enhancing the digestibility of nutrients. At the same time, it can also provide animals with probiotics and prebiotics, and can increase the content of free amino acids with antioxidant properties, such as histidine, serine, valine and lysine, and the concentration of phenolic compounds. In this context, the soybean meal was fully fermented under aerobic conditions with Bacillus subtilis as the bacterial source, and the fermented soybean meal used in this study was obtained. This product removed most of the anti-nutritional factors and degraded most of the molecular protein, increased amino acid content, has a good potential to replace fish meal. At present, the research on soybean meal fermented by Bacillus subtilis mainly focuses on the hydrolysis of soybean protein and the improvement of antioxidant activity, and there is very little research on the effect of fish feeding. Largemouth bass (Micropterus salmoides) has been introduced into China since it has been widely cultivated due to its delicious meat, few intermuscular spines, and strong disease resistance. After years of breeding development, it has become one of the important species of freshwater aquaculture in China. Therefore, the largemouth bass was used as the research object, and proved the feasibility of Bacillus subtilis fermented soybean meal to replace fish meal through experimental research, hoping to provide some references for the optimization of largemouth bass feed formula and the sustainable development of artificial breeding industry. To explore the effects of fermentation of soybean meal instead of fish meal on the growth, lipid metabolism, serum non-specific immunity and intestinal flora of juvenile largemouth bass, fish meal was replaced with fermentation of soybean meal at 0, 10%, 20%, 30%, and 40%, respectively. Crystal amino acids were used to balance the difference between methionine and lysine in each group, and 5 isonitrogen equal energy diets (CP47%, GE19 MJ/kg) referred to as FM, FSM10, FSM20, FSM30 and FSM40 were used to feed Largemouth bass with an initial weight of (22.05±0.09) g with three replicates for 58 days. The results showed that there were no significant differences in the weight gain rate (WGR), specific growth rate (SGR), condition factor (CF), hepatosomatic index (HSI) and viscerosomatic index (VSI) among groups (P>0.05), but soybean meal significantly increased the feed coefficient ratio (FCR) (P<0.05). The survival rate (SR) and apparent dry matter digestibility (ADDM) of the FSM40 group were significantly lower than other groups (P<0.05). The serum triglyceride (TG) levels of the FSM30 and FSM40 groups were significantly lower than the FM control group (P<0.05), and the serum total cholesterol (T-CHO) level of the FSM40 group was significantly lower than the FM control group (P<0.05). The activities of lysozyme (LZM) and superoxide dismutase (SOD) in the FSM40 group increased significantly (P<0.05). The Alpha diversity indexes (Sobs, Shannon, Simpson, Ace and Chao) of of intestinal flora had no significant difference among groups (P>0.05), but the number of OTUs were different. The main phyla in intestine of fish were Tenericutes, Proteobacteria and Fusobacteria in soybean meal groups; the dominant genera were Mycoplama, Plesiomonas and Cetobacterium. Compared with the FM control group, the relative abundances of Tenericutes and Mycoplama in the FSM20 group increased significantly (P<0.05), and the relative abundances of Proteobacteria and Plesiomonas significantly reduced (P<0.05). Overall, under the experimental conditions, the replacement of 20% fish meal with fermented soybean meal in largemouth bass feed did not have a negative impact on the growth, lipid metabolism, serum non-specific immunity and intestinal flora diversity of largemouth bass juveniles, and It significantly reduced the relative abundance of harmful bacteria including the Plesiomonas in intestine, so the appropriate ratio of fermented soybean meal to replace fish meal was 20%.