Abstract: To consume renewable energy,the Circulating Fluidized Bed( CFB) boiler units can play great role in the deep peak regulation and operation flexibility with its strong peak regulation capability. However,due to the safety limitation of fluidization in dense phase zone under ultra-low load operation,the total amount of primary air cannot continue to decrease,which destroys the inherent reductive characteristics of staged primary and secondary air and causes more nitrogen oxides to be formed. In the meanwhile,the furnace outlet temperature is much lower than the temperature window of selective non-catalytic reduction( SNCR),which causes efficiency drop of denitration system in the separator. Flue gas recirculation technology is a kind of low-load NOxcontrol technology suitable for CFB boilers. In this paper,the comparison of operating performance before and after the flue gas recirculation modification in a 330 MW subcritical CFB boiler unit was introduced. The results show that under low load conditions,flue gas recirculation technology can significantly reduce the primary air volume while maintaining the safety of fluidization in the dense zone,strengthen the reducing atmosphere and reduce the temperature in the dense zone,delay the combustion in the furnace,and significantly improve the flue gas temperature at the furnace outlet,which greatly avoid the reduction of SNCR efficiency in the separator. At last,a reasonable operation control strategy was provided for the corrosion in the flue gas recirculation system.