Abstract: Sulfation reaction process of red mud and limestone were investigated by thermogravimetric analysis method. Phase composition of desulfurization product and calcined sample microstructure were examined by XRD, mercury intrusion porosimeter and SEM respectively. Desulfurization mechanism of red mud was discussed, which provided useful data for red mud as substitute for limestone used for coal desulfurization agents. It is shown that the possible desulphurization index of red mud is higher than that of limestone under the same condition. Calcined red mud has more middling size pores and larger specific surface areas than calcined limestone, which enhances desulfurization reaction rate. Moreover, red mud includes more ferrite and alkali metal salt compared with limestone. At low temperature they can enhances lattice defect of desulfurization agents and the diffusion ability of Ca2+ and O2-. This improves speed of surface chemical reaction and final reaction. At high temperature ferrite and alkali metal may form fused mass with dicalcium silicate together. This reduces specific surface area and porosity of red mud, thus, desulfurization activeness of red mud is weakened. Alkali earth metal carbonate may be added when red mud is used for desulfurization agents in higher temperature boiler.