In this study, we investigated the immunomodulatory effects of Bacillus velezensis GV1 polysaccharides (BPS) on gut microbial regulation in a cyclophosphamide (CTX)-induced immunosuppressed mouse model. BPS treatment significantly restored immune function, as evidenced by improvements in spleen and thymus indices and increased cytokine expression levels, including IL-6, TNF-α, IFN-γ, and IL-2. Additionally, BPS administration effectively alleviated the CTX-induced histopathological damage to the colon, enhanced tissue repair, and maintained epithelial integrity. The beneficial effects of BPS were associated with the selective modulation of the gut microbiota, and notably enhanced beneficial bacterial taxa, including PAC001112_g, PAC001074_g (Muribaculaceae), Monoglobus, PAC001500_g (Peptococcaceae), and PAC002153_g (Lachnospiraceae). Metagenomic analysis further revealed key metabolic pathways affected by BPS, such as folate biosynthesis, fructose and mannose metabolism, and pentose and glucuronate interconversion, involving essential enzymes such as xylulokinase, triosephosphate isomerase (TIM), and fructan beta-fructosidase. Collectively, these findings indicate that BPS effectively alleviates CTX-induced immunosuppression by modulating gut microbiota composition, underscoring its potential as a valuable supplementary or functional agent with immune-enhancing effects.