Objective：To investigate the change of bacterial composition in the gut of Crohn’s disease（CD）patients in comparison with the healthy subjects and the effects of Bifidobacterium longum（B. longum） on differentiation of peripheral blood mononuclear cells（PBMCs） to CD25+Foxp3+Treg cells and secretion of interleukin（IL）-10，IL-12，and transforming growth factor（TGF）-β of PBMCs in patients with CD. Methods：Fecal samples were obtained from active CD patients（n=19）and healthy subjects（n = 20）. Bacterial microbiome was analyzed by high-throughput sequencing of 16S rRNA. PBMCs obtained from CD patients were co-cultured in vitro with B. longum，B. longum supernatant，B. longum medium and PBS. The proportion of CD25+Foxp3+Treg cells in the peripheral blood was determined by flow cytometry. Concentrations of IL-10，IL-12 and TGF-β in culture supernatant were measured by ELISA. Results：We observed that bacterial microbiota was skewed in CD，with an increased proportion of Proteobacteria and a decreased proportion of Firmicutes and Actinobacteria at the phylum level，and an increased proportion of Escherichia Shigella，Streptococcus，and Veillonella，while a decreased proportion of Faecalibacterium，Gemmiger，Bifidobacterium，Ruminococcus，Roseburia and Fusicatenibacter at the genus level. Compared with B. longum medium and PBS，B. longum and B. longum supernatant showed a significant higher induction capacity of the anti-inflammatory cytokine IL-10 and TGF-β，while did not remarkably elicit the production of pro-inflammatory cytokine IL-12. The B. longum supernatant induced the highest IL-10/IL-12 ratio and the frequency of peripheral blood CD25+Foxp3+Treg cells in PBMCs，followed by the B. longum. Conclusion：Both B. longum and B. longum supernatant display anti-inflammatory capacities and properties to induce the peripheral blood CD25+Foxp3+Treg cells differentiation. The metabolite of B. longum shows a promising role in treating the inflammatory bowel disease.