Objective To explore the efficacy and safety of converting the triple immunosuppressive regimen of tacrolimus (Tac) + mycophenolate mofetil (MMF) + prednisone (Pred) to a quadruple regimen of low-dose sirolimus (SRL) + low-dose Tac + MMF + Pred at 3 to 6 months after expanded criteria donor (ECD) kidney transplantation. Methods A single-center, retrospective, donor-matched controlled study included 22 ECD kidney transplant recipients from September 2021 to June 2024. Two recipients from the same donor kidneys were respectively assigned to the SRL group and the conventional triple regimen control group. The main outcome measures were the differences in serum creatinine (Scr), estimated glomerular filtration rate (eGFR), and adverse events before the regimen conversion and after conversion during the 1, 3, 6, and 12-month follow-up. Results There were no statistically significant differences in baseline characteristics between the two groups. In the SRL group, Scr decreased and eGFR increased starting from 3 months after conversion, and this was superior to the control group starting from 6 months(all P < 0.05). There were no statistically significant differences in the incidence of rejection reactions, pulmonary infections, hyperlipidemia and proteinuria between the two groups after conversion and during the 12-month follow-up (all P > 0.05). Conclusions For ECD kidney transplant recipients, converting the triple regimen to the SRL quadruple regimen at 3 to 6 months after transplantation may improve the function of the transplanted kidney without increasing the risk of adverse events.