Combustion experiments were performed in a bubbling bed combustor to explore the effects of various oxygen concentrations (21% to 40%), temperatures (850 °C to 950 °C), and mixing ratios (0% to 30%) on the formation of NO gas. In order to correspond to different combustion stages, the generated NO were distinguished as volatile-NO and coke-NO, respectively, and the total amount and the conversion rate of NO were analyzed. The results indicated that NO comes mainly from fixed carbon combustion, and an increased oxygen concentration in the environment could produce more NO during the combustion process, regardless of the blending of biomass. The temperature increase promoted the conversion of nitrogen in the anthracite and accelerated the release of volatiles from the pine powder. During mixed combustion with increasing temperature, the volatiles formed had diluted the environmental oxygen concentration, which led to suppressed NO emissions. In addition, increasing the blending ratio also resulted in decreased NO emissions due to the large amount of intermediates released by the burning pine powder that induced a deoxidation effect on the NO emission.