The CaSO4 reductive decomposition is an interesting issue in both the reduction zone of fluidized bed combustors (FBCs) for coal combustion and the fuel reactor of chemical-looping combustion (CLC) system. Under CO or H2 atmosphere, CaSO4 is reduced to CaS and CaO, together with the releases of gas sulfides, which causes environmental pollutions. To lessen sulfur release, it is important to figure out the chemical stability of CaSO4 reductive decomposition. Thus, the chemical stability of CaSO4/CaS/CaO under CO or H2 atmosphere was studied in consideration of SO2, COS and H2S emissions. The results show that regions I (VI), II (V), and III (IV) are the stability fields of CaSO4, CaS and CaO, respectively. The range for CaO stability is increasing with reaction temperature and partial pressures of CO2 and H2O. Within the reaction temperature range of 800 and 1,000 °C, when the CaSO4-CO-H2 reaction system reaches the triple equilibrium point, the main gas sulfur released is SO2, followed by H2S, while COS generation is much smaller. In a real reaction system, when the values of real PH2/PH2O (PCO/PCO2), PSO2, PH2S (PCOS), PH2O (PCO2) and T fall into Region I (IV), or II (V), the final product should be CaSO4 or CaS, and the sulfur release from CaSO4 reduction can be controlled.