This study investigates the properties and hydration mechanism of gypsum-based composite cementitious materials using flue gas desulfurization gypsum (FGDG), fly ash (FA), ground granulated blast furnace slag (GGBS), and ordinary Portland cement (OPC). Orthogonal experiments were conducted to optimize the composition of the materials, with the optimal ratio found to be 20% OPC, 56% FGDG, 19.2% FA, 4.8% GGBS, and a water-to-binder ratio (W/B) of 0.55. The hydration products and microstructure of the materials were analyzed using XRD, SEM, and MIP techniques to understand the synergistic hydration mechanism. The results show that FGDG and OPC synergistically react, generating AFt under the action of sulfate ions, and that FA and GGBS are activated by alkali-salt excitation, producing a large amount of cementitious materials that fill pores in the gypsum crystal structure, forming a dense microstructure. The study also found that the hydration reaction of FGDG and OPC, along with the synergistic stimulation of FA and GGBS, significantly improves the mechanical properties and water resistance of the composite materials. The results provide insights into the comprehensive utilization of industrial solid waste and the application of gypsum-based cementitious materials in non-structural components.This study investigates the properties and hydration mechanism of gypsum-based composite cementitious materials using flue gas desulfurization gypsum (FGDG), fly ash (FA), ground granulated blast furnace slag (GGBS), and ordinary Portland cement (OPC). Orthogonal experiments were conducted to optimize the composition of the materials, with the optimal ratio found to be 20% OPC, 56% FGDG, 19.2% FA, 4.8% GGBS, and a water-to-binder ratio (W/B) of 0.55. The hydration products and microstructure of the materials were analyzed using XRD, SEM, and MIP techniques to understand the synergistic hydration mechanism. The results show that FGDG and OPC synergistically react, generating AFt under the action of sulfate ions, and that FA and GGBS are activated by alkali-salt excitation, producing a large amount of cementitious materials that fill pores in the gypsum crystal structure, forming a dense microstructure. The study also found that the hydration reaction of FGDG and OPC, along with the synergistic stimulation of FA and GGBS, significantly improves the mechanical properties and water resistance of the composite materials. The results provide insights into the comprehensive utilization of industrial solid waste and the application of gypsum-based cementitious materials in non-structural components.