To promote large-scale resource utilization of coal gangue and ecological restoration in mining areas, this study developed an artificial soil for slope rehabilitation using coal gangue (CG), slag powder (SP), and desulfurization gypsum (DG). Through multi-ratio experiments, the relationships between physicochemical properties (bulk density, porosity, pH, electrical conductivity, and nutrient content) and water-holding performance (water retention, loss rate, capillary/non-capillary porosity, and evaporation) of composite substrates were analyzed to select the optimal mix. Pot experiments evaluated the ecological suitability of three plants—Elymus dahuricus, Lolium perenne, and Festuca elata—in the substrate. Results showed that reducing coal gangue content decreased bulk density, increased porosity, and enhanced water-holding capacity. At m(CG):m(SP):m(DG) = 70:15:15, water-holding porosity rose by 35.97%, pH reached 7.03, water retention increased by 73.05%, and loss rate decreased by 70.18%, demonstrating