In recent years， the potential anti-tumor effects of metformin have attracted widespread attention in the field of cancer treatment. This article summarizes the research progress of metformin in the treatment of malignant tumors，finding its potential application in the treatment of malignant tumors in the digestive system （biliary tract cancer，gastric cancer，esophagus cancer，colorectal cancer，pancreatic cancer，liver cancer） and reproductive system （prostate cancer，ovarian cancer，breast cancer， cervical cancer），non-small cell lung cancer，renal cell carcinoma，and melanoma. Metformin can inhibit the proliferation of tumor cells and extend the overall survival of patients. Its mechanisms of action include，but are not limited to，inhibiting the activity of mitochondrial complex Ⅰ，activating adenosine monophosphate-activated protein kinase/p53 signaling pathway，and blocking the cell cycle. Additionally，the combined use of metformin with chemotherapy drugs has shown potential for reducing toxicity and enhancing efficacy. It can enhance the sensitivity of biliary tract cancer，ovarian cancer，and melanoma cells to chemotherapy drugs， improve the drug resistance of gastric and colorectal cancer cells to chemotherapy，and reduce the toxic reactions of breast cancer patients during chemotherapy. Metformin is also used as an immunomodulator，applied in the immunotherapy of patients with esophagus cancer，colorectal cancer，cervical cancer，non-small cell lung cancer，and melanoma.