Abstract: Taking the early PC utility boiler as examples, an operating mode of optimizing air distribution in furnace is put forward to lower NO_X emission. It has the advantage of simply operation and low cost. The experimental result shows that NO_X emissions will increase with the total air rate, however, the economy of the boiler decreases, so the total air rate must take consideration of both the economy and NO_X emissions of the boiler. Under the constant total air rate, optimizing the secondary air and over fire air may further decrease NO_x emissions with the lower secondary air rate in the middle of burners and the larger secondary air rate in upper and lower layers in burners. Under the premise of constant secondary air ratio, NO_X emissions decrease with increasing the over fire air ratio. The operating mode in this paper can effectively decrease NO_X emissions from 908 mg/m³ to 790 mg/m³.