In this paper, a numerical investigation and experimental study was used to research the effect of a power plant 600MW supercritical four walls tangentially fired boiler furnace over fire air opening size on the inside furnace NOx concentration distribution and the results coincide. There are four cases in all. The influence and formation of NOx that was produced by pulverized coal furnace during combustion processes were analyzed. The research was proved that the over fire air has great effect on the concentration distribution of NOx in the furnance.