The effect of the ratio of secondary air to separated overfired air (SOFA) on the generation of NO_x in a 330 MW oppose-firing coal fired boiler under 180 MW low load condition was investigated by numerical method.The results show that the NO_x generation is not a linear function of the main combustion zone air ratio α_A/α_D and SOFA β; it presents a parabolic shape and has an optimal value.The other parameters variation curves in the furnace have a similar trend.When the SOFA accounts for about 24% of the total volume and the main combustion zone air ratio is about α_A/α_D=0.78/0.8,the NO_x production can be reduced by about 20% compared with other operation conditions.