Abstract: The karst rocky desertification is serious in the karst areas of southwest China. Its unique surface and underground dual-hydrogeological structure have led to the water and soil loss in the surface karst zones, which is manifested as slope soil loss and the phenomenon of water and soil leakage to the ground. In order to reveal the law and mechanism of underground leakage of soil on karst slopes, the physical simulation tests and CFD-DEM numerical simulations are used to analyze the law and characteristics of soil particle movement in the process of underground leakage of soil on slopes. The applicability and accuracy of the CFD-DEM numerical model are verified by comparing and analyzing the underground runoff and sediment yield process and the proportional characteristics of the runoff and sediment yield of the physical tests and the numerical model. The results show that the movement speed of soil particles in different layers on the slope surface gradually increases from the top to the bottom of the slope, and it tends to be consistent throughout the slope. From the profile, the movement speed of soil particles changes from the surface layer > the middle layer > the bottom layer at the beginning of rainfall to the same speed at the end of rainfall. With the rainfall process, the soil on the slope is first eroded from the surface runoff outlet and underground leak, and then forms erosion rills on the slope, which further aggravates the process of erosion and finally develops to the overall slope slip. During the rainfall process, the contact number between particles on the slope first significantly decreases and then slightly increases. The contact number between particles on the lower edge of the crack and the slope gradually increases. The contact force between particles and the slope in the Y direction is small, indicating that the contact force between the particles and the slope is weak during the rainfall process.