Abstract: In order to clarify the influence of the location of the external dumping on the stability of the open-pit slope, a two-dimensional theoretical model of composite slope is established, and the formula for calculating the safety factor of end-slope stability under the action of the dumping site is derived, and the mechanism of the influence of the distance between the external dumping and the end-slope on the safety factor of the slope is expounded. In addition, a three-dimensional numerical analysis model is established by using FLAC3D software, and the slope stability of the composite slope of the external dumping and the side slope of a mine in China is analyzed. The failure mechanism and characteristics of composite slope in open pit coal mine are expounded. By studying working conditions under the distance of 50 m, 70 m, 90 m, and 110 m between the dump and the side of the mine, it is found that the safety factor of the slope increases with the increase of the distance. The safety factors of the models with distances of 50 m and 70 m are relatively low. The safety factors have been improved obviously after the distance exceeds 90 m. At the same time, according to the calculated safety factors and displacement data of the simulated monitoring points, the safe distance between the dumping site and the side wall of the mine is 110 m.