This paper investigates the impact of flue gas desulfurization (FGD) gypsum's crystal modifier on the characteristics and microcosmic mechanism of α-high strength gypsum. The results demonstrate that all three crystal modifiers can convert FGD gypsum to α-high-strength gypsum. Citric acid (CA) has the most significant influence on α-high-strength gypsum, and the prepared α-high-strength gypsum is short columnar, with an aspect ratio in the range of 1–3, and has a faster setting time, a larger specific surface area, and a smaller standard consistency, higher compressive strength, greater surface hardness, and smaller crystal particle size. The initial setting time of the α-high-strength gypsum manufactured with CA crystal modifier was decreased by 36% compared to the blank sample, the final setting time was lowered by 37.5%, and the water consumption of the standard consistency was reduced by 8%. The maximum strength is 32 MPa after 2 h, the absolute dry compressive strength is up to 38 MPa, and the surface hardness is improved by 24.43%.