Diabetic retinopathy (DR) is one of the most common complications of diabetes. Angiotensin-converting enzyme inhibitor is thought to play an important role in preventing and treating retinal diseases in animal models of DR. The aim of the present study was to investigate the role of angiotensin-converting enzyme inhibitor (ACEI, captopril) in the treatment of patients with non-proliferative DR.Three hundred and seventeen type 2 diabetic patients (88.05% of participants) without or with mild to moderate non-proliferative retinopathy were randomly divided into captopril group (n = 202) and placebo group (n = 115). All subjects received 24-month follow-up. General clinical examinations, including blood pressure and glycated hemoglobin, as well as comprehensive standardized ophthalmic examinations were performed. Color fundus photography and optical coherence tomography (OCT) were used to grade diabetic retinopathy and detect macular edema respectively.The levels of blood pressure and glycated hemoglobin in the two groups of patients remained within the normal range during the entire follow-up and no significant difference was found between the initial and last visits, suggesting that ACEI drugs play a protective role on the DR patients independent of its anti-blood pressure role. DR classification showed that 169 eyes (83.66%) remained unchanged and the DR grade of 33 eyes (16.34%) increased in captopril group, while 84 eyes (73.04%) remained unchanged and the grade of 31 eyes (26.96%) increased in placebo group (P = 0.024). Captopril treatment improved macular edema in 55.45% eyes, which was significantly higher than the 37.39% improvement in placebo group (P = 0.002). No significant difference was found in the visual acuity between the two groups (P = 0.271).Captopril can improve or delay the development of DR and macular edema, which can be used in the early treatment of DR patients with type 2 diabetic mellitus.