Abstract: Based on the 200 000 t/d Shenyang Degremont Anoxic Oxic process (SDAO) of Guodian Shenyang North Wastewater Treatment Plant, the primary settling tank and the sludge recovery zone in SDAO process were transformed to anaerobic zone and pre-anoxic zone in modified A²O process. The pollutants removal performances of the modified A²O process were investigated. The results showed that the modified A²O process effluent of TN, NH₄⁺-N, TP and COD_Cr was 11.8, 1.34 0.17 and 31.7 mg/L, when influent temperature, COD/TN ratio, COD_Cr, TN and TP were around (12.2±0.5)℃, 4.0-6.0, 200 mg/L, 35.9 mg/L and 3.05 mg/L with adding sodium acetate (COD_Cr of 50 mg/L). Modified A²O process removal performance of COD_Cr and NH₄⁺-N was similar to SDAO process. However, TN removal ratio in the modified A²O process could increase from 35.5% to 67.8% compared with the SDAO process. Moreover, TP removal ratio in the modified A²O process could increase to 94.3% by contrast with incompetent TP removal performance of SDAO process. In the condition without reconstruction of pre-anoxic zone and anaerobic zone it was able to enhance nitrogen and phosphorus removal performance obviously.