Activated semi-coke, an economical carbonaceous material, is employed in the removal of SO2 from simulated flue gas of an industrial power plant. Activation by four commonly used agents, including CO2, KOH, ZnCl2, and H3PO4, is studied in detail, which demonstrates that the sample treated with KOH at high temperature presents the best performance on removal of SO2. Further investigation on the physical and chemical properties reveals that both optimized pore structure and increased amount of active sites of activated semi-coke could contribute to the high desulfurization capacity. The semi-coke activated with KOH is selected to discuss possible mechanisms of the adsorption and desorption processes. Performances with variation of desulfurization temperatures evidence that physically adsorbed SO2 can transform into chemically adsorbed SO, which is significantly affected by temperature. Desulfurization behaviors under different flue gas compositions show that oxidation of SO2 to SO3 plays a key role in SO2 removal. By the analyzing desorption behavior of the samples, a reactive intermediate, C(O) complex, is proposed to be generated by dissociated chemisorption of O-2 onto the surface of activated semi-coke, which serves as the dominating active site in oxidation of SO2. Besides, activated semi-coke exhibits several favorable properties in this study that could offer the prospect for further application in industrial desulfurization.