In this paper, the transformation of gypsum to anhydrite phase in titanium gypsum-H2SO4-H2O system under atmospheric pressure was studied. The effects of reaction time, sulfuric acid concentration and temperature on the transformation of gypsum to anhydrite were explored. The results show that increasing reaction temperature, sulfuric acid concentration and reaction time are beneficial to the transformation of gypsum to anhydrite. When gypsum is converted to anhydrite, the anhydrite do not change after prolonging the reaction time. When the temperature is 70 ℃ and 80 ℃, gypsum and anhydrite coexist in the system, and there are obvious differences in the microstructure, gypsum is flake, and anhydrite is rectangular plate. When the temperature rises above 90 ℃, gypsum is completely transformed into anhydrite, and its microscopic appearance is rectangular plate. Under atmospheric pressure, in the range of 50 ℃ to 100 ℃, gypsum-anhydrite transformation occurs in the system of titanium gypsum-H2SO4-H2O system, gypsum is directly dehydrated to anhydrite, no intermediate products are formed in the process.