In order to investigate the influence of second air distribution on NOx formation, this paper numerically simulated the NOx formation rule by using Ansys for the swirl pulverized coal boiler in a 660 MW ultra-supercritical power plant. The numerical results agree well with actual conditions, which verify the effectiveness of the numerical simulation results. The main results are summarized as follows: the burner in furnace is the mainly region of NOx formation； the concentration of NOx between the first layer and second layer of combustion zone is higher； the concentration of NOx decreases along the furnace height； with the increasing of burnout-air, the mean concentration of NOx at each cross-section zone decreases along the height of boiler, and the mean concentration of NOx at the burner outlet decreases, but the decreasing rate is more and more slow.