Abstract: Since the construction and production of soft rock open-pit mines in Xilin Gol League and Hulunbuir League of Inner Mongolia, tens of thick loess-based waste dumps have been formed. Its mechanical properties determine the poor bearing capacity of the loess foundation, which is very unfavorable to the stability of the dump and leads to the increasingly serious problem of slope stability control. In view of the actual conditions of Shengli east No.2 open-pit coal mine, combined with the distribution characteristics of the dump base load, the calculation method of the soft basement base bearing capacity was proposed by improving the traditional construction base bearing capacity algorithm, and the ultimate pile height of the south dump was discussed from the angle of the base bearing capacity. Based on the principle of landslide back analysis, the shear strength parameters of basement soil are determined. By analyzing slope stability, the spatial parameters of slope are proposed. The results show that the ultimate height of the dump is not limited by the traditional foundation bearing capacity because of the triangular lateral load and the small slope angle of the dump. When the north slope height of the south dump of Shengli east No.2 open-pit coal mine is 80 m and 120 m, the corresponding stable slope angle is 15° and 14°, and the corresponding flat plate width is 53 m and 60 m, respectively.