Bifidobacterium longum is a dominant strain colonized in the human intestine and is closely related to human intestinal health. Adhesion is a prerequisite for colonization of Bifidobacterium longum in the intestine， so it is particularly important to study the adhesion mechanism of Bifidobacterium longum. bif is a type of extracellular adhesion factor， but its specific adhesion in the intestine has not been clarified. In this experiment， homologous recombination technology was used to construct bif adhesion knockout mutants to study the adhesion of bif gene. Firstly， the bif adhesion gene and PBR322 in the C16 gene of Bifidobacterium longum were used as templates to amplify the upstream and downstream homology arms and tet resistance genes of the bif adhesion gene， and then the upstream and downstream homology arms and tet of the bif adhesion gene， respectively. The resistance gene was ligated to pMD19-t simple vecter， and the intermediate vector pMD19-up， pMD19-down， pMD19-tet was double-digested， and the recovered fragment was ligated to PUC18 to form the knockout vector PUC18-up-tet-down. The host strain was introduced into the strain by electroporation， and the Bifidobacterium longum bif adhesion gene-deficient strain was obtained by homologous recombination screening. The results of colony PCR and sequencing showed that the construction of bif adhesion gene knockout strain was successful， which laid a foundation for further study on the adhesion of bifidobacteria bif gene.