The invention provides a method for desulfurizing and treating flue gas from a boiler of a thermal power plant. The method includes steps of 1), boosting pressures of the flue gas from an electrostatic dust collector by the aid of a booster fan, cooling and dehalogenating the flue gas by the aid of alkali liquor, and evaporating the alkali liquor and process water to reduce the temperature of the flue gas and allow the temperature of the flue gas to be lower than 100 DEG C; 2), allowing the cooled and dehalogenated flue gas to flow through a flue gas heater, then feeding the flue gas into a desulfurizing tower via an oxidization flue, to be more specific, feeding the flue gas into the desulfurizing tower from a flue gas input zone and carrying out reaction on SO2 in the flue gas and alkali liquor in the desulfurizing tower; 3), demisting and treating the flue gas after desulfurization reaction is carried out on the flue gas, removing liquid drop carried in the flue gas and separating clean flue gas from the flue gas. The alkali liquor at the step 1) is ash rinsing water from the boiler and solution with slag dissolved in water. The oxidation flue is connected with a strong oxidation system, and the desulfurizing tower is sequentially provided with a demister, a spray layer, the flue gas input zone and an alkali liquor pond zone from top to bottom. The method has the advantages that hydrogen halide in the flue gas can be effectively removed by the aid of the method, the desulfurization efficiency of the desulfurizing tower can be improved, and the service life of the desulfurizing tower can be prolonged.