The numerical simulation method was used to study the material flow and NO_x emission characteristics of a 75 t/h circulating fluidized bed (CFB) boiler. The formation and distribution of the pollutant NO_x at the furnace outlet was studied by calculating the trajectory of fuel particles and changing the excess air coefficient. At the same time, the dynamic effects of sophericity of particles on the fluidization were given. The numerical simulated results show that the excess air coefficient of the boiler furnace should be selected between 1.10 and 1.15, and the optimized combustion condition is further determined. The ratio of primary air to the second air should be 1:1, and the excess air factor should be chosen as 1.12. The results provide a practical reference to the combustion optimization in a CFB boiler, and are valuable for engineering applications.