Using a 330 MW opposed-fired pulverized coal boiler at a thermal power plant as research object, numerical analysis was conducted on the flow, heat transfer and NO_x emission under different air distribution ratios at low load operation of 150 MW. The simulation results show that when secondary air ratio and overfire air ratio increase, the trend of NO_x formation is not linear. An optimum air distribution method with the highest combustion efficiency and the lowest NO_x emission can be obtained. In order to verificate the optimal effect of this optimum condition and the feasibility of actual operation，the on-site experiments were carried out at three numerical working conditions. The experimental results show that the mass concentration of NO_x at the outlet is reduced by 20% and the boiler efficiency is improved by 0.41% at the optimum air ratio compared with the original operating conditions. It proves the economy and feasibility of the optimization scheme and provides optimization ideas for similar engineering problems.