Objective To evaluate the capacity of renal functional reserve (RFR) in elderly patients with chronic kidney disease (CKD) stage 3. Methods A total of 123 CKD stage 3 patients hospitalized in our department from January 2020 to December 2020 were enrolled as the study subjects. The formula based on serum cystatin C and serum creatinine was used to calculate their glomerular filtration rate (eGFR), and then they were divided into CKD3a group (n=75) and CKD3b group (n=48). RFR was calculated by the difference of eGFR before and after intravenous amino acid infusion. The general clinical data and RFR level were compared between the 2 groups. The incidences of acute renal injury (AKI) and rapid decline of renal function were compared among the patients with different RFR levels during 1 year of follow-up. SPSS statistics 21.0 was used for data analysis. According to the data type, two independent samples t-test, one-way ANOVA or Chi-square test was employed for intergroup comparison. Results The RFR level was 21.61±7.13,12.80±9.33, and 7.23 ± 5.84 ml/(min·1.73m²), respectively for the patients aged 65 to 75,76 to 85 and >85 years, and there were significant differences in pairwise comparison (P<0.05). The RFR level of the CKD3a patients was significantly higher than that of the CKD3b patients [22.43±5.33 vs 9.85±3.40 ml/(min·1.73m²), P<0.05]. The RFR level of CKD3a patients was mainly 10%~20%, accounting for 46.67% (35 cases); the main component of RFR level was <10% in the CKD3b patients (21 patients, 43.75%). The CKD3a patients complicated with diabetes had obviously lower RFR level when compared with those with CKD3a alone [20.06±3.57 vs 26.28±8.15 (ml/min·1.73m²), P<0.05]; and similar result was seen between the CKD3b diabetes patients and those with CKD3b alone [7.57±4.65 vs 10.07±5.31 ml/(min·1.73m²), P<0.05]. In 1 year′s follow-up, the CKD3 patients with RFR<10% had notably higher incidences of AKI [25.64% (10/39) vs 16.33% (8/49) and 14.29% (5/35)] and rapid decline of renal function [28.21% (11/39) vs 12.24% (6/49) and 17.14% (6/35)] than the CKD3 patients with RFR<20% and RFR>20%. Conclusion With the increase of age, the RFR level in elderly CKD3 patients is in a downward trend. The CKD3b patients have lower RFR level than the CKD3a patients. Elderly CKD3 patients with lower RFR level suggest a higher risk of acute renal events.