Abstract: For a 220 t/h high temperature and high pressure circulating fluidized bed（CFB） biomass-fired boiler, the fuel properties existed differences with the designed fuel, the combustion of gas phases in the furnace was unstable, and the emission of NO_x presented on a high-level. In order to reduce NO_x emissions and improve the boiler efficiency, optimized and modified measures had been done such as optimization of biomass fuel pretreatment and feeding system, modification of staged combustion, reducing of the cyclone separator resistance. The results show that optimization and modification has achieved as significant results, the release of NO_x concentration can be controlled under 100 mg/m~3, the CO concentration can be controlled under 2×10^-3, and the boiler efficiency increases by 5.1%.