Circulating fluidized bed (CFB) ash has good volcanic ash properties, desulfurization building gypsum was used as the main cementitious material, carbide slag was used as an alkali activator, CFB ash and fly ash were used as mineral additives to study the effect of CFB ash on the fluidity, setting time, compressive strength and expansion rate of desulfurization building gypsum. The hydration products and microstructure were analyzed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and other methods. The results show that adding CFB ash can effectively improve the mechanical property and water resistance of desulfurization building gypsum, but it has a negative impact on its workability and expansiveness. After composite modification with 20% (mass fraction) CFB fly ash and 5% (mass fraction) carbide slag, the absolute dry compressive strength of desulfurization building gypsum at 1 and 28 d is 20.53 and 22.37 MPa, which increases by 28.6% and 35.9% compared to desulfurization building gypsum based samples, respectively. The calcium silicate hydrate (C-S-H) gel and ettringite (AFt) generated from the hydration of CFB ash are wrapped and filled in the crystal surface and voids of dihydrate gypsum, making the internal structure of the hardened body more compact, thus improving the mechanical property and water resistance of desulfurization building gypsum.