Early reports indicate that metformin, a clinical drug administered to treat type 2 diabetes mellitus (T2DM), was found to be associated with a better prognosis of cancer. The objective of this study was retrospectively analyzed the effect of metformin on the outcomes of Chinese breast cancer patients with T2DM. A total of 3757 primary invasive breast cancer patients who underwent surgery from January 2010 to December 2013 were enrolled. According to the medication treatment, all the patients were divided as non-diabetes group, metformin group and insulin group. The follow-up data for disease-free survival (DFS) and overall survival (OS) were obtained from 3553 patients (median follow up of 85 months) and estimated with the Kaplan-Meier method followed by a log-rank test. Multivariate Cox proportional hazards regression model was applied. The results showed that there was a significant survival difference among non-diabetes group, metformin group and insulin group, 5-year DFS was 85.8%, 96.1%, 73.0%, and 5-year OS was 87.3%, 97.1%, 73.3% respectively (P < 0.05). Prognostic analysis showed metformin was significantly associated with better DFS and OS. Our results suggested that metformin may have a good effect on the survival of invasive breast cancer patients with T2DM.