The invention relates to a method for desulfurizing flue gas of a combustion boiler. The method comprises the following step of: conveying flue gas into a desulfurizing tower through a boiler induced draft fan; spraying ammonia water from the bottom of the desulfurizing tower; washing and cooling the flue gas with ammonia water for absorbing SO2; discharging defogged clean flue gas through a wet chimney on the top of the tower; oxidizing the ammonia water which absorbs ammonium sulfite formed by SO2 in the flue gas into ammonia sulfate with the air introduced from the bottom of the desulfurizing tower; cooling the flue gas by using the ammonia sulfate solution and concentrating the ammonia sulfate solution simultaneously to obtain an ammonia sulfate slurry with the solid content 5-15 percent; conveying the slurry with the solid content 5-15 percent into the ammonia sulfate process through an ammonia sulfate discharging pump; dehydrating with a cyclone to form an ammonia sulfate slurry with the solid content 40 percent; making the ammonia sulfate with the solid content 40 percent enter a centrifuge for performing solid-liquid separation to form wet ammonia sulfate containing 4 percent of water; returning mother solutions in cyclone and the centrifuge to an absorbing tower; and drying the wet ammonia sulfate containing 4 percent of water with a drier to obtain an ammonia sulfate product of which the water content is less than 1 percent. The method has the advantages of high desulfurizing rate, no secondary pollution, and capabilities of recovering sulfur dioxide and meeting the requirement of circular economy.