Air leakage in the coal pulverizing system can reduce the hot air flow into the coal mill, deteriorate the ventilation process, so that the coal mill output and the power consumption for coal milling are increased. The air leakage to the system will finally enter the hearth , as a result the temperature in the boiler drops, the radiation heat transfer reduces and the convection heat transfer rate increases, and in the same time the stability of combustion become poor. Due to the cold air entering the boiler via the coal pulverizing system, under the condition of invariable total air volume, the amount of air through the air preheater will be reduced, as a result the exhaust fume temperature rises and the boiler thermal efficiency will decline.