The NOx emission of circulating fluidized bed (CFB) boilers in China is more and more stringently controlled due to the updated national emission regulation (NER). Based upon the theory of CFB combustion and previous research results related to NOx, formation mechanism, a technical route was proposed by means of re-specifying the fluidization state in CFB boilers, to achieve the ultra-low NOx emission without installing the deNO(x) equipment additionally. The main method is to increase the solid circulating rate and decrease the bed inventory as well as the mean size of bed material, which would influence the bed temperature and oxidizing/reducing atmosphere distribution, and thereby benefit for the reduction of NOx emission. Besides, the reasonable control-of bed temperature and air supply should be considered. This technique has been demonstrated to be effective in three CFB units with different capacities (150 t.h(-1) 260 t.h(-1) and 560 t.h(-1)) and burning different types of coal. No obvious decrease of combustion efficiency was found in the field tests. The technical route provides a feasible approach to further control the NOx emission for CFB boilers. (C) 2016 Elsevier B.V. All rights reserved.