The invention discloses a method for oxidizing CaSO3 in desulfurized fly ash in an alkaline condition. The method comprises the following steps of: (1) adding 2 to 10 times water by weight to the desulfurized fly ash, and uniformly stirring to obtain desulfurized fly ash slurry; (2) slowly adding H2O2 and/or persulfate solution with weight concentration being 10 to 30% of to the desulfurized fly ash slurry, wherein the molar mass ratio of H2O2 and/or persulfate to CaSO3 is 1- 2: 1; and (3) stirring and reacting mixtures of the desulfurized fly ash slurry, H2O2 and/or persulfate solution for 0.5 to 4 hours. According to the oxidization method, the pH of the solution does not need to be adjusted and metallic ions do not need to add, so that the reaction conditions are mild, and as a result, the oxidization cost is reduced, and the corrosion to the equipment is reduced; and the method has the characteristics of short reaction time, high efficiency, etc.