A mathematics model is proposal for simulating desulfurization process in a CaO activation reactor of a limestone injection FGD system. The effective factors, including particle motion, water evaporation, particle impact, and aqueous sorbent reaction, have been considered in this model. By use of calculating results and experimental results from a 3000 N .m(3)/h pilot plant experimental system of CaO activation reactor, the effect of flue gas temperature, particle size, water injection and Ca/S molar ratio on desulfurization efficiency is investigated in detail. It is found that desulfurization efficiency rises with the increase of Ca/S molar ratio, the amount of water injection and the decrease of particle size, flue gas temperature. A method to improve desulfurization efficiency in the CaO activation reactor is also given in this paper. The above results are a help for semi-dry FGD system design.