Abstract: As a renewable carbon-neutral energy, biomass is an excellent substitute for fossil energy. The rational utilization of biomass energy is an important way to achieve the "carbon peaking and carbon neutrality" goal in China. Among the various utilization technologies of biomass, co-firing biomass in coal-fired boilers can not only reduce the consumption of fossil fuels and the emissions of pollutants such as NO_x and SO_2,but also reduce the risk of the corrosion of the boiler tail heating surface caused by pure burning of biomass, which is an effective measure to improve the efficiency of energy utilization. In this paper, the 660 MW pulverized coal boiler was taken as the research object, and the numerical simulation software Fluent was used to study the co-combustion process of biomass and pulverized coal, the realizable turbulence model was used to describe the gas-phase turbulence, the eddy dissipation model was used to calculate the combustion of volatiles, and the influences of biomass injection location and particle size on combustion process and nitrogen oxide emission were investigated. The calculation results show that the addition of biomass can promote the combustion of pulverized coal to a certain extent, whereas the NO_x emission is reduced. When biomass is injected from the primary air vent at the bottom layer, it can release the reducing components such as volatile matter earlier, so that the NO_xemission reduction effect is better. The NO_x mass concentration in the flue gas at the furnace outlet is reduced by 18.4% from 298.92 mg/m~3 in the original working condition to 243.97 mg/m~3. Although the change of biomass particle size has little effect on the flue gas temperature, O_2 and CO volume fraction in the furnace, the excessive particle size will lead to the reduction of biomass particle burnout rate, thus reducing the required oxygen. As a result, the pulverized coal is easier to burn completely, and the NO_x mass concentration in the outlet flue gas increases.