On the basis of experimental data, the numerical simulation was carried out for a600 MW swirling opposed utility boiler, using ANSYS 15.0 to investigate the effects of different over-fire air (OFA) ratios and different ratios of the upper OFA to the lower on combustion, and analyze the mutual restriction of reducing the incomplete combustion loss and NOx emission.The study shows that the incomplete combustion loss increases and NOx emission reduces with the OFA ratio increasing;when the ratio of the upper OFA to the lower becomes larger, the reducing zone increases, the combustion of unburned carbon is unfoworable, but the air classification is strengthened, which makes the NOx emissions decreased.By comprehensive consideration of the influences, 20%25% was chosen as the optimal OFA ratio, and 3∶2 was chosen as the optimal two layers OFA ratio.