The invention relates to a method for preparing alpha semi-hydrated gypsum from dihydrate gypsum. The method comprises the following steps: adding dihydrate gypsum and a crystal modifier into a mixed acid water solution containing sulfur acid and phosphoric acid to carry out crystal transformation, carrying out solid-liquid separation, returning a part of liquid phase into a crystal transformation tank for crystal transformation, and conveying another part of liquid phase to an acidolysis tank for use, wherein the acidolysis tank is used for preparing phosphoric acid by decomposing phosphorite by virtue of sulfuric acid; adding washing water to wash a solid phase, and introducing the washing water into the liquid phase for use. The washed solid phase can be prepared into alpha semi-hydrated gypsum powder through drying and can be prepared into gypsum products such as gypsum boards, gypsum blocks, gypsum components and the like by directly adding water without drying. According to the method, the alpha semi-hydrated gypsum is prepared in the mixed acid water solution containing sulfur acid and phosphoric acid, procedures including desulfuration, removal of impurity and the like on raw gypsum can be omitted, furthermore, mixed acid can be recycled, and the obtained alpha semi-hydrated gypsum product is high in strength and stable in quality.