Abstract: Yimin Open-pit Mine belongs to the open pit mine of water-rich soft rock slope. The system of water rich coal rock slope is characterized by low strength, large water filling, poor permeability, complex occurrence of evolution weak layer, and prone to landslide. By analyzing the geological conditions of Yimin Open-pit Mine, the seepage mode of groundwater in inner dump is determined. Based on the typical geological section, SEEP/W seepage analysis model is established to simulate the groundwater seepage in the inner dump. Through the simulation of the current internal drainage and the designed internal drainage, it is concluded that the current internal drainage is relatively stable, but there is still landslide risk in rainy season.