ObjectiveTo systematically evaluate effect of metformin on prognosis of colorectal cancer patients with type 2 diabetes mellitus (T2DM).MethodsThe PubMed, Embase, Web of Science, Cochrane Library, CNKI, VIP, Wangfang, etc. databases for cohort studies over the past 10 years were systematically searched. The relationship between the metformin and the prognosis in the colorectal cancer patients with T2DM was assessed with RevMan 5.3 software.ResultsA total of 12 articles with 13 694 patients were included in this study, of which 9 069 patients treated by the metformin (metformin group) and 4 625 patients treated by the other antidiabetic agent (control group). The results of meta-analysis showed that the metformin group had higher over survival [HR=0.74, 95%CI (0.66, 0.82), P<0.000 01] and cancer-specific survival [HR=0.77, 95%CI (0.69, 0.86), P<0.000 01] as compared with the control group. The sensitivity analysis of the heterogeneity showed that the research results did not be changed when the study which might cause heterogeneity was excluded. ConclusionMetformin treatment could improve prognosis of colorectal cancer patients with T2DM and improve overall survival and cancer-specific survival.