The original design scale of a municipal sewage treatment plant in Sichuan was 10×10⁴ m³/d, and the effluent complied with the First-Level A discharge standard of GB 18918－2002. The main process used an improved DE oxidation ditch. Practice has shown that the operation of the factory cannot meet the standard stably and has limited potential for improvement. With the increasing environmental requirements, it is now necessary to meet the discharge requirements of urban sewage treatment plants in the "Water Pollutant Discharge Standards for Minjiang and Tuojiang River basins in Sichuan Province" (DB 51/2311－2016). Through the analysis of the original process and operational practice, the measure of transforming the DE oxidation ditch into a multi-stage AO and adding a deep treatment process is adopted, that is, upgrading the oxidation ditch into a multi-stage AO + secondary sedimentation tank + denitrification filter + sodium hypochlorite disinfection standard process. The results showed that when the average COD, TN, and TP of the influent were 258, 41.8 and 3.8 mg/L, the treated effluent decreased to 14.8, 7.2 and 0.1 mg/L, with corresponding removal rates of 94%, 83%, and 97%, respectively. The effluent met the new standard requirements. This technical renovation project fully utilized the original structure, with low investment, land saving, and high treatment efficiency, which can be used as a reference for similar wastewater treatment plants to improve standards and operate.