The semi-dry desulfurization ash of iron ore sintering flue gas cannot be effectively utilized because it contains a lot of unstable calcium sulfite. Oxidation modification of desulfurization ash is an important means to realize its large-scale or high-value utilization. The results show that the calcium carbonate in the desulfurization ash is the main factor limiting its oxidation modification. Calcium carbonate and calcium sulfite cover each other, which reduces the specific surface area of calcium sulfite oxidation reaction and limits its oxidation rate. Two weak acids, L(+)-ascorbic acid and glacial acetic acid, were used to remove calcium carbonate. Glacial acetic acid with a concentration of 0.4 g/g could increase the content of calcium sulfite to 81.17% with the maximum increase of 69.71%. The effect of different factors on the oxidation rate of calcium sulfite in desulfurization ash was studied by subcritical hydrothermal heterogeneous oxidation method. The results showed that the oxidation rate of calcium sulfite was significantly increased after removing impurities. The oxidation rate of calcium sulfite can be increased by increasing the initial pressure, the reaction temperature and time, and reducing the solid-liquid ratio. When the reaction temperature was 140℃, the reaction time was 30 min, the initial pressure was 2 MPa, the initial solid-liquid ratio was 1:30 and the rotating speed was 300 r/min, the oxidation rate of calcium sulfite was 98.72%, while the oxidation rate of desulfurized ash without impurity removal was only 78.77% at 180℃(other conditions were the same). In this work, the desulfurization ash was oxidized rapidly at low temperature. From the micro morphology point of view, compared with the desulfurized ash without impurity removal, the calcium sulfite after impurity removal was easier to grow along the radial direction, which had preliminary conditions for the preparation of calcium sulfate whiskers with a large aspect ratio, which was of great significance for the preparation of calcium sulfate whiskers.