The condition of preparing α-hemi hydrate gypsum from FGD gypsum at atmospheric pressure is easy to achieve. It acquires atmospheric pressure and low heating temperature. This paper introduces two different methods of preparing α-hemi hydrate gypsum, aqueous salt solution method and dry dehydration. Use the microscopic characterization of α-hemi hydrate gypsum to identify the product. Observe the crystal of product by stereo microscope and characterize the compounds with Fourier transforms infrared spectroscopy. Investigate the factors’ effects between the two methods.