Abstract: A new grid method was adopted. The simulation has been divided into two cases:non air leakage and existing air leakage. In both cases, numerical simulation of flow, heat transfer, combustion and pollution release law in a 300 MW tangentially coal fired boiler under different conditions are performed by FLUENT software. The results showed that with air leakage of furnace, flue gas temperature rises, CO₂ concentration decreases and NO concentration increase, which leads to the increase of flue gas loss,the difficulty of CO₂ recovery and emission amount of pollutions.With the increase of O₂ concentration, the impact of air leakage grew in furnace, the economy of boiler operation reduced significantly, and even the security risks also increased.