Abstract: Cement production is a major contributor to global CO₂ emissions, motivating the research and development process of low-carbon cementitious materials using industrial solid waste as raw materials. This study investigated a metallurgical slag−gypsum composite cementitious material as a partial substitute for ordinary Portland cement, with emphasis on the hydration mechanisms, hydration products, and strength development rules of the metallurgical slag−gypsum composite cementitious material. A response surface methodology mixture design was used to optimize the proportions of steel slag, ground granulated blast furnace slag, and desulfurized gypsum. Ten mixtures were prepared and tested for uniaxial compressive strength at 7 d and 28 d, and selected typical mixtures were characterized using X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy. The response surface model exhibited excellent fitting performance, with the coefficients of determination （R²） for the models at curing ages of 7 d and 28 d being 0.987 and 0.837, respectively. The early strength of the material was primarily governed by the interaction between DG and slag, whereas the later strength was dominated by the regulating effect of the overall proportioning components. The hydration process exhibited distinct stage characteristics: The early stage was centered on the formation of ettringite, while the later stage relied on the growth of calcium-aluminosilicate gel to achieve matrix densification. The optimized proportioning group （33.5 % SS, 55 % slag, and 11.5 % DG） exhibited the best mechanical properties at 7 d and 28 d. The research results indicate that the metallurgical slag−gypsum composite cementitious material can provide the mechanical properties required for mine backfilling and other low-strength construction applications, while effectively reducing environmental impact.