Abstract: The NO_x emission of the circulating fluidized bed (CFB) boiler is not only affected by the fuel properties, but also closely related to the combustor performance and operation conditions. It is important to study the relationship between NO_x emission and several design or operation parameters, which is also the key to realize the de-NO_x combustion in engineering for a CFB boiler. Under a specific boiler load and for a selected fuel type, several engineering methods were proved to have significant effects on the reduction of NO_x emission, such as improving the performance of circulating loop including improving cyclone efficiency, appropriately decreasing the size of feeding coal and limestone particles, adjusting the air staging and controlling the oxygen content in furnace, controlling the bed temperature and bed pressure drop in reasonable range, improving the uniformity of air and coal injection, etc., This paper reviewed the NO_x formation and reduction mechanism under CFB combustion conditions, and summarized the effects of several design or operation parameters on the NO_x emission. The principles of some main de-NO_x combustion technologies were also analyzed. Finally, the key scientific issues and the future research directions related with NO_x emission of CFB combustion technology were discussed.