More than 80% of open-pit coal mines in China are located in northern regions, and the mechanical properties and stability of loose soil-rock mixtures in waste disposal sites are significantly affected by freeze-thaw effects. This article takes the external dumping site of the Baorixile open-pit coal mine in the northern high-altitude region of the Inner Mongolia Autonomous Region as the research object. Through on-site investigation and sampling, indoor triaxial tests (confining pressures of 100 KPa, 200 KPa, and 300 Kpa; moisture contents of 18%, 21%, and 24%), numerical simulation, and other methods, the mechanical properties of soil-rock mixtures in the dumping site under different freeze-thaw cycle conditions were tested to reveal the specific influence of the number of freeze-thaw cycles on the mechanical properties of soil-rock mixtures. Using the discrete element software PFC, the microstructural changes in soil-rock mixtures formed by freeze-thaw cycles were studied, and the deformation mechanism and slip mode of loose slopes in waste disposal sites under different freeze-thaw cycle conditions were explored. The relationship between the number of freeze-thaw cycles and slope stability was clarified. The following conclusions can be drawn: the compressive strength of soil-rock mixtures decreases as a quadratic function with increasing freeze-thaw cycles; as the number of freeze-thaw cycles increases, the internal cracks of the soil-rock mixture model increase exponentially; and as the number of freeze-thaw cycles increases, the stability of the slope in the dumping site decreases significantly, and this stability also decreases with an increase in dumping height.