A three-dimensional numerical investigation is presented on the coal combustion and NOx emission in a supercritical 600 MW wall fired utility boiler fed with lean coal. The distributions of velocity, temperature, and species were obtained using a finite volume method and some validated submodels. The influence of air staging condition, i.e., overfire air (OFA) ratio and OFA port position, on combustion and especially on NOx emission was studied. The temperature of the burner zone and the carbon content in fly ash increased with increasing OFA ratio, while the NOx emission decreased with increasing OFA ratio. An increase of the OFA port position resulted in a decrease of NOx emission and an increase of carbon content in fly ash. In addition, high burnout rate of pulverized coal and low NOx emission can be achieved in the present boiler, in which OFA was injected from Port 3 with a ratio of 34.2%. This study provides new insights into the physical and chemical processes in a wall fired utility boiler fed with lean coal and also illustrates a method to reduce NOx emission.