Abstract: In recent years,with the increasingly strict requirements of air pollutant emission standards,most circulating fluidized bed units have added with desulfurization and denitrification facilities outside the furnace,and the combined mode of inside and outside the furnace is used to control pollutant emission concentration.However,due to the the characteristics of CFB boiler such as high thermal inertia and desulphurization lagging in furnace,the key equipment failures and poor operation adjustment control,abnormal environmental parameters are easy to occur in the start-up and shutdown of the unit,which will affect its safety and environmental protection operation.Based on the discussion of the pollutant control process and technical route of CFB Unit,the key influencing factors of pollutant original emission and the key points of removal process were analyzed,the abnormal phenomena and causes of SO_2,NO_x and soot were studied,and the countermeasures were put forward from the aspects of equipment failure,operation adjustment,automatic control,start-up and shutdown of the unit.The results show that the abnormal SO_2 of CFB unit is mainly caused by the large change of sulfur content in coal,coal blockage in coal bunker and poor return of material from the feeder,etc.The abnormal NOxis usually caused by low SNCR denitration efficiency during unit startup and shutdown,and CEMS system failure is the key reason for abnormal soot.Aiming at the abnormal environmental parameters,measures such as reducing the fluctuation of pollutant discharge concentration by strengthening the coal-burning quality and admixture management,using the anti-blocking technology of coal bin to solve the problem of coal blockage,eliminating the inconvenience of returner in unit load-lifting,strengthening the maintenance of pollutant control and monitoring system,automatically increasing the logic and feed-forward of pollutant estimation,setting the AGC adjustment rate and automatic blocking in different load sections,can effectively avoid abnormal environmental parameters in unit operation. In the start-up and shutdown of the unit,lime can be used as desulfurizer and flue gas recirculation can be used to control oxygen content,so as to further reduce the abnormal environmental parameters of the unit.