Desulfurization of flue gas by ozonation aided by the catalysis of a metallic ion was investigated experimentally. The results showed that the removal efficiency of SO2 was only 30% at a 1:1 molar ratio of O3/SO2 without the addition of a catalyst. However, the efficiency was substantially increased when a metallic ion was added to the absorption solution at the same ratio of O3/SO2. In particular, the performance was optimal using a Mn2+ catalyst. Under the optimal condition given as the absorption liquid temperature of 20 °C, gas flow rate of 10 L/min, molar ratio of 1.0 for O3/SO2, and Mn2+ concentration of 6.4 × 10–4 mol/L, the removal efficiency of SO2 was 99%. A reaction mechanism of SO2 with ozone in the liquid-phase oxidation catalyzed by Mn2+ was proposed. The advantage of ozonation desulfurization technology is that SO2 is directly transformed into a valuable product, sulfuric acid, which provides favorable environmental and economic benefit.