The desulfurization characteristics of limestone under O-2/CO2 atmosphere were studied by a drop-tube furnace test rig. The experimental results show that high CO2 concentration makes the calcination reaction of limestone slow and that the pore structure of calcined limestone is more suitable for high-temperature desulfurization under O-2/CO2 atmosphere. A desulfurization mechanism for limestone under O-2/CO2 coal combustion atmosphere is proposed based on the experiments. The system keeps on producing nascent CaO continuously. At the same time, the diffusion resistance through CaSO4 product layer is reduced because CO2 is formed at the CaO/CaCO3 interface before complete limestone decomposition into CaO. The above two factors make CaO keep on a high sulfation rate for a long time under O-2/CO2 atmosphere. So the high Ca conversion of limestone can be reached under O-2/CO2 atmosphere. There is an optimal desulfurization temperature for limestone in O-2/CO2 atmosphere as in air. And the optimal desulfurization temperature and corresponding desulfurization efficiency increase with the CO2 concentration in O-2/CO2 atmosphere. It is implied that the O-2/CO2 atmosphere is beneficial to limestone desulfurization at higher temperature.