α high strength gypsum was prepared using phosphogypsum as raw material. The effects of autoclave temperature, autoclave time, water content in the slurry and thickness on 2h flexural strength and dry compressive strength of α high strength gypsum were investigated by orthogonal test. Transformation process of phosphogypsum to α high strength gypsum was studied. The results show that α30 high strength gypsum can be obtained under the following conditions: autoclave temperature of 130℃, autoclave time of 6h, water content in the slurry of 30%(by mass), thickness of 15mm with addition of crystal modifier of 0.13%. Transformation of phosphogypsum to α high strength gypsum nucleus follows the solution crystallization mechanism through autoclave. Without modifier, the nucleus changes into acicular crystal at last. Under the influence of the modifier, growth rate of the nucleus in the c axis is slowed down, and then growth rate in every direction reaches equilibrium. Thus the nucleus change into hexagonal columnar crystal. At the same time the addition of the modifier can improve crystallinity of the product.