The decomposition behavior of calcium sulfate and the influence of various additives at high temperature were investigated in this paper. The results show that SiO2, ZnO and the compound of SiO2-Fe2O3 can effectively restrain the decomposition of calcium sulfate, attributed mainly to the formation of new products Ca2SiO4 and Ca2Fe2S2O3. A high sulfur retention rate can be achieved by incorporating compound Al2O3-CaCO3 into CaSO4 at the molar ratio of CaCO3∶Al2O3∶CaSO4=3∶3∶1, and XRD proves that large amounts of sulfoaluminates occur in the slag.