The boiler is additionally provided with an air preheater at the end of the flue gas, and the air is preheated to a certain temperature with the heat carried by the flue gas of the boiler. The heated air enters the combustion room again, which not only accelerates the drying, ignition and combustion processes of the fuel, ensures the stable combustion in the boiler, but also improves the average temperature level in the boiler, thereby strengthening the radiation heat transfer in the boiler and effectively improving the thermal efficiency of the boiler.