Abstract: To solve the problems of fluidization safety and ultra-low NO emission of supercritical CFB boiler under ultra-low load,flue gasrecirculation technology was introduced. Taking a supercritical 350 MW CFB boiler as the research object,a flue gas recirculation auxiliarypeak-shaving system after semi-dry desulfurization was built. Based on the data of field operation,the effects of operating parameters suchas the average bed temperature,separator inlet temperature,exhaust temperature,NO,CO mass concentration of outlet flue gas,combustible content of fly ash and bottom ash,steam-water parameter at 30%-60% load were studied under the flue gas recirculation. The resultsshow that under the coordination of flue gas recirculation,the final emission of NO at 30%-60% load is less than 50 mg/ m after SNCR,the original NO emission is less than 50 mg/ m at 30% load; the bed temperature is reduced by 15-22 ℃ ,the boiler exit flue temperatureincreases 10-13 ℃ ,and the primary and reheat steam temperature is above 558 ℃ . With the decrease of boiler load,the carbon content offly ash and bottom slag increases,and the mass concentration of CO does not increase significantly. The NO is reduced about 15 mg/ mby SNCR arranged at secondary air; the minimum change rate of load rise/ fall is 1.23% Pe; the boiler keeps dry operation above 30%load,the maximum deviation of water wall temperature is 55 ℃ at 30% load,the maximum deviation of overheated and reheated heatingsurfaces is 55 and 47 ℃ respectively,and no over-temperature appears.