As a kind of solid wasteflue gas desulfurization (FGD) gypsum can be used to prepare calcined gypsum by calcining at high temperature to realize the resource utilization of solid waste. FGD gypsum was used as raw materialcalcium oxide and aluminum sulfate were added as composite crystal modifiersand calcined gypsum was prepared by calcining at 170 ℃ for 2 h. The effects of the composite ratio and dosage of composite crystal modifiers on the properties of calcined gypsum were studiedand the composite crystal conversion mechanism was revealed. The results show that when the content of composite crystal modifier is 1% (mass fraction) and the composite ratio of calcium oxide and aluminum sulfate is 1∶1 (mass ratio)the prepared calcined gypsum has the best property. The gypsum block is dense after hydrationand the hydration products are in the form of alternating short columns or fibers. The 2 h flexural and compressive strength of calcined gypsum are 3.6 and 9.7 MPathe dry flexural and compressive strength are 68 and 235 MPawhich meets the requirements of grade 3.0 calcined gypsum in Calcined Gypsum (GB/T 9776-2022).