Abstract: The reversible reaction of gas desulfurization by CaO was investigated using a thermal balance and a fixed-bed reactor. The results show that there is an optimum desulfurization temperature，i.e. the decomposition temperature of Ca(OH)2 in the absence of carbon dioxide, and the steam and hydrogen have different effects on the reversible reaction of desulfurization. Increasing the concentration of steam benefits the reverse reaction both in thermodynamics and kinetics, which decreases the efficiency of desulfurization. However, the effect of hydrogen on the reverse reaction at high temperature is negative. As the concentration of hydrogen increases to a certain value, the reverse reaction becomes a single reaction instead of multiple reactions, and the efficiency of desulfurization can be determined by the thermodynamic properties of reaction (CaO + H2S = CaS + H2O). In addition, the possibility to use the CaS/ H2O reaction to deal with the waste CaS is discussed.