Abstract: The modified A²/O process was used to treat urban sewage, and the effect of optimized operation of primary sedimentation tank (PST) on nitrogen and phosphorus removal were investigated in the specific engineering example. Six different operation conditions were adopted in the test, namely, the flow distribution ratio of raw water entering PST to that directly entering the biological tank stepping over PST were 10∶0, 8∶2, 6∶4, 4∶6, 2∶8 and 0∶10, respectively. The results showed that the optimized operation of PST had significant influence on the concentration of total nitrogen in the effluent and the efficiency of biological phosphorus removal, but had little influence on the ammonia nitrogen concentration. The phenomenon of denitrifying phosphorus removal existed in the experimental process, and the denitrifying phosphorus removal rate was positively related to the flow distribution ratio of raw water stepping over PGT directly entering the biological tank. Among the six different conditions, the more optimal flow distribution ratio of raw water entering PST to that directly entering the biological tank stepping over PST was 6:4. In the later stage of this condition, the average concentration of total nitrogen in the effluent was 8.79 mg/L, which was 33.9% lower than the original condition; the concentration of ammonia nitrogen was lower than 0.50 mg/L, and the average concentration of total phosphorus in aerobic area was 0.34 mg/L, which met the first level A standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002); the power consumption was 0.337 kW·h/m³, which was only 5.31% higher than the original operation condition, with a small increase. Compared with the original condition, although the power consumption slightly increased, the controllability of the effluent water quality was further strengthened, and it was significantly better than the discharge standard of the first level A.