Metformin is a classic hypoglycemic drug. It reduces fasting and postprandial hyperglycemia in diabetic patients by reducing liver glucose synthesis and intestinal glucose absorption. It is mainly aimed at obese patients with type 2 diabetes. Recent studies have shown that metformin can also inhibit the proliferation, migration，invasion of tumor cells and inhibit tumor angiogenesis.Meanwhile it also can induce apoptosis and induce cell differentiation. Metformin affects the signal transduction pathway and energy metabolism of tumor cells and immune cells through adenosine monophosphate protein kinase (AMPK)⁃dependent which is divided into direct and indirect ways and non⁃AMPK⁃dependent mechanisms, participates in improving the tumor immune microenvironment of hypoxia and immunosuppression, improves metabolism and immune function by regulating intestinal flora, and exerts an anti⁃tumor effect and then achieve the purpose of treating tumors.There have been many clinical trials, which have adopted different treatment measures in combination with metformin, and have achieved certain results. This article reviews the effects of metformin on tumor cells, tumor microenvironment, intestinal flora and the related clinical trials of metformin against tumor.