This study investigates the properties and hydration mechanism of gypsum-based composite cementitious materials, aiming to achieve resourceful, large-scale, and high-value utilization of bulk industrial solid wastes such as flue gas desulfurization gypsum (FGDG), fly ash (FA), and ground blast furnace slag (GGBS). Orthogonal experimental methods were used to prepare these materials, and the optimal composition was determined to be 20% OPC, 56% FGDG, 19.2% FA, and 4.8% GGBS, with a water-to-binder ratio (W/B) of 0.55. The hydration products and micro-morphology were analyzed using XRD, SEM, and MIP techniques to elucidate the synergistic hydration mechanism. The results show that the hydration reaction between FGDG and OPC can be synergistic, and C-A-H further generates AFt under the action of SO42−. This process also plays a role in the alkali-salt joint excitation for FA–GGBS, generating a large amount of cementitious materials and filling the pores of the gypsum crystal structure, forming a dense microstructure. The study provides insights into the comprehensive utilization of industrial solid wastes 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, aiming to achieve resourceful, large-scale, and high-value utilization of bulk industrial solid wastes such as flue gas desulfurization gypsum (FGDG), fly ash (FA), and ground blast furnace slag (GGBS). Orthogonal experimental methods were used to prepare these materials, and the optimal composition was determined to be 20% OPC, 56% FGDG, 19.2% FA, and 4.8% GGBS, with a water-to-binder ratio (W/B) of 0.55. The hydration products and micro-morphology were analyzed using XRD, SEM, and MIP techniques to elucidate the synergistic hydration mechanism. The results show that the hydration reaction between FGDG and OPC can be synergistic, and C-A-H further generates AFt under the action of SO42−. This process also plays a role in the alkali-salt joint excitation for FA–GGBS, generating a large amount of cementitious materials and filling the pores of the gypsum crystal structure, forming a dense microstructure. The study provides insights into the comprehensive utilization of industrial solid wastes and the application of gypsum-based cementitious materials in non-structural components.