In recent years, China has established many large water conservancy projects in the western mountainous areas. However, dam foundation excavation produces a huge amount of waste rock mixture. A soil-rock mixture with unique structural characteristics can easily cause geological disasters, such as collapse, landslide, and debris flow following an earthquake, rainfall, and engineering disturbance. Therefore, research on the strength and shear characteristics of the soil-rock mixture is beneficial to avoiding soil-rock mixture disasters. In this study, the soil-rock mixture in the slag dump site of Jinping Hydropower station is taken as the research object. The influence of different fillers on the gravel soil shear characteristics is investigated through a large-scale triaxial test of coarse-grained soil. The slope stability of the slag dump site is analyzed using Midas GTS. Then, the most unfavorable sliding surface of the slag dump site is discovered and the slope stability coefficient under the most unfavorable conditions is calculated. The results lay a scientific foundation for the design and construction of a slag dump slope.