Harmless treatment of CaS from coal gas desusulfurization and other industries is of very importance for the environmental protection. In this paper, the experimental investigation of calcium sulfide regeneration with steam at atmospheric pressure was carried out using TGA, fixed bed reactor and SEM-EDX analysis. The results show that the reaction mechanism of CaS with steam varies with temperature. The reaction occurs obviously at the temperature above 630℃ and the dominant products include CaSO4 (or CaSO3) and H2S, as the temperature is over 850℃, the primary products become CaO, SO2 and H2S. The CaS regeneration reaction will strongly depend on the temperature and the data over 900 oC can be fitted by the Shrinking-Core Model (SCM).