Gastrointestinal microbiota has an important impact on physiological functions of human body. In recent years, the importance of gastrointestinal microbiota in tumorigenesis has emerged. Aberrant location and proportion of microbiota can canot only carcinogenesis of local epithelia, but also tumorigenesis in submucous tissues or even distant organs. Certain microbes can produce genotoxin or generate reactive oxygen species to caDNA damage and affect DNA damage repair, which leads to genetic instability and induces cell transformation. Moreover, gastrointestinal microbiota is able to affect the function of host immune system and form an immunosuppressive microenvironment, while some other pathogenic bacteria can cachronic inflammation and may be involved in tumor immune escape. Based on the above, gastrointestinal microbiota is oncogenic and closely linked to tumorigenesis.