It is of great significance to accelerate the hydration process of steel slag and stimulate the gelation activity to improve the comprehensive utilization rate of steel slag. Using slag and desulfurized gypsum as composite exciters, based on the design method of cross-experiment, the composition of composite cementitious materials was optimized, the comprehensive properties of composite cementitious materials were analyzed, and X-ray diffraction (XRD), scanning electronic microscopy (SEM), fourier transform-infrared spectroscopy (FT-IR), and other test methods were used to characterize. The composition and structure of composite cementitious materials were analyzed, and the mechanism of synergistic optimization of composite cementitious materials was revealed. The cross-experiment results of the mixing ratio of the composite cementitious material show that, steel slag 30%, slag 60%, desulfurization gypsum 10%, water-cement ratio is 0.5, 28 d compressive strength is 42.4 MPa, and other property indexes meet the requirements of GB/T1346-2011 Test methods for water requirement of normal consistency, setting time and soundness of the Portland cement. Through a number of testing methods, it can be judged that the slag is continuously hydrolyzed under the excitation of desulfurized gypsum, which shortens the early stage of hydration induction and prolongs the induction period, improves the degree of early hydration reaction, releases a lot of heat, and generates ettringite (AFt) and C-S-H gels. The increase of the pH value of the slurry promotes the hydration reaction of the steel slag and accelerates the conversion of the slurry from the liquid phase reaction to the solid phase reaction, which proves that there is a synergistic reaction between the raw materials.