Abstract: The changes in intestinal flora are usually associated with different gastrointestinal diseases, and intestinal flora homeostasis can enhance immune tolerance and regulate intestinal immune balance.Previous studies have found that the increase of the relative abundance of Bacteroides fragilis (B.fragilis) in Bacteroides intestinalis can significantly enhance the expression of intestinal regulatory T cells (Treg) and anti-inflammatory cytokines, thus alleviating intestinal inflammation.However, the mechanism of B.fragilis regulating intestinal immunity is still unclear.In this study, an acute colitis model was constructed by giving 3% DSS in drinking water solution to SPF-grade C57BL/6 mice for 7 days, and exogenous supplementation B.fragilis was given to mice by gastric gavage to study its regulatory effect on intestinal immunity and its mechanism of action.The results showed that B.fragilis could improve the intestinal flora disorder in mice with colitis and increase the content of short-chain fatty acids (SCFAs), the main metabolite of the intestinal flora.By extracting mouse tissue lymphocytes, naive CD4⁺ T cells, and liposome-modified siRNA knockdown mouse Smad3, it was further discovered by flow cytometry that B.fragilis induced the expression of intestinal Treg cells and related cytokines through the TGF-β/Smad3 signaling pathway, which enhanced intestinal regulatory immunity and alleviated colitis.It was also found that B.fragilis activated TGF-β by increasing the expression of reactive oxygen species (ROS), thus inducing Treg cell differentiation and playing an immunomodulatory role.