Background: Chronic obstructive pulmonary disease (COPD) is often combined with type 2 diabetes mellitus (T2DM) in clinical, and with poor prognosis. In recent years, research shows that inflammation is a common characteristic of COPD and T2DM. T-helper 17 cell (Th17)/regulatory T-cell (Treg) balance controls inflammation and may be important in the pathogenesis of COPD combined with T2DM patients. This study investigated the characteristics of Th17, Treg and related inflammatory factors in COPD combined with T2DM patients and the potential mechanism. Methods: Application of flow cytometry technology, real-time fluorescent quantitative PCR and ELISA to detect the changes in peripheral blood of Th17 and Treg number and the expression of key transcription factors and related cytokines in COPD combined T2DM patients were performed. Results: Patients with COPD combined with T2DM revealed significant increase in peripheral Th17, Th17 related cytokines (IL-17A, IL-17F, IL-21, IL-23, IL-6) and transcription factor (ROR gamma t) levels and significant decrease in Treg, Treg-related cytokines (IL-10, TGF beta 1) and transcription factor (Foxp3) as compared with patients with COPD, T2DM and healthy controls. Conclusion: Th17/Treg functional imbalance exists in patients with COPD combined with T2DM, indicating a potential role of Th17/Treg imbalance in the formation and progression of COPD combined with T2DM.