The invention provides a method for simultaneously desulfurating, denitrating and decarbonizing coal-fired flue gas. The method comprises the following steps: dedusting and cooling the coal-fired flue gas containing SO2, NOx and CO2; leading the gas into an adsorbent for saturated adsorption at the adsorption temperature of 50-100 DEG C and the adsorption pressure of 50-10000Pa; evacuating the coal-fired flue gas, desorbing the adsorbent by means of pressure reduction or temperature rise, and then repeatedly using; leading the desorbed gas into ammonia water for absorption treatment, wherein, the mass ratio of the ammonia water to the mixed gas containing SO2, NOx and CO2 is 2-6:1, the absorption temperature is 30-50 DEG C, and the absorption pressure is 5000-12000Pa; and recycling the gas to generate ammonium sulfate, ammonium nitrate and ammonium carbonate. The method can achieve simultaneouse desulfuration, denitration and decarbonization, and has small investment, low cost and low energy consumption; and the method can be widely applied to purification treatment of SO2, NOx and CO2 in the waste gas discharged from fixed fire coal sources such as thermal power plants, smelting plants, oil refinery, chemical plants and the like.