Abstract: Numerical simulation was conducted to the low NO_x combustion retrofit for a 330 MW subcritical opposed firing boiler, so as to study the influence of overfire air jet form on its penetration capability through high-temperature viscous flame in the furnace, the carbon burnout rate and the NO_x emission under retrofit conditions, on the premise of same overfire air ratio, constant distance from top coal burner to the nozzle and of fixed nozzle area. Results show that the circular nozzle direct flow overfire air has the strongest penetration capability, followed by the rectangular nozzle direct flow overfire air, and the overfire air in the form of outer cyclone inner direct flow has the weakest penetration capability. Highest CO emission is generated under the condition with circular nozzle direct flow overfire air due to its lowest comprehensive level of covering area of jet roots on the horizontal cross section in the furnace and its lowest air layer thickness along flue gas flow path. Lowest unburned carbon exists in the fly ash in the over-fire air jet form of outer cyclone inner direct flow because of its high-intensity turbulent mixing effect during later jet period. The overfire air in three different jet forms discussed play little role in NO_x emission.