The prevention and treatment for gastrointestinal tumors have become an important public health issue due to the high morbidity and mortality. Microflora in the gastrointestinal tract usually participate in the metabolism and immune response to maintain body homeostasis. Recent studies have found that a variety of bacteria in the gastrointestinal tract play an important role in the occurrence and development of gastrointestinal tumors. Bacteria could induce tumors by virulence factors, biofilm, metabolites and other factors. The efficacy of chemotherapeutic agents could be affected as well. However, the involvement of bacteria in tumorigenesis remains unclear. Therefore, this paper reviews the mechanisms of gastrointestinal tumors induced by bacteria, thereby providing the theoretical basis for early prevention and treatment.