To investigate the value of serum cystatin C for the evaluation of renal function damage in patients with proliferative diabetic retinopathy (PDR). The clinical data of PDR patients treated in ophthalmic wards of Beijing Tongren Hospital from July 2019 to January 2021 were retrospectively analyzed. The estimated glomerular filtration rate (eGFR) was calculated by the abbreviated Modification of Diet in Renal Disease (MDRD) study equation. Based on the eGFR level, the patients were divided into no obvious renal function damage group [≥ 60 ml·min⁻¹·(1.73 m)⁻] and renal inadequacy group [0.05). The systolic blood pressure, blood urea nitrogen, creatinine, uric acid, triglyceride, cholesterol and cystatin C were higher in the renal inadequacy group than those of no obvious renal function damage group (all <0.05). However, the glycosylated hemoglobin, albumin and high-density lipoprotein were lower in the renal inadequacy group (all <0.05). Multivariate logistic regression analysis showed that serum creatinine (=1.189, 95%: 1.101-1.284, <0.001) and cystatin C (=3.175, 95%: 1.272-7.923, =0.013) were independent risk factors for renal inadequacy in PDR patients. The ROC curve showed that cystatin C had a predictive value for renal inadequacy, with an area under the curve (AUC) of 0.966 (95%: 0.952-0.979). Meanwhile, the cutoff value was 1.315 mg/L, and the Youden index was 0.826, with the sensitivity of 94.3% and specificity of 88.4%. Serum cystatin C can be used as an important laboratory indicator to evaluate the renal impairment in PDR patients.