In order to scientifically evaluate the slope stability of open-pit mine down dip soft rock, the stability of the soft rock slope on the down dip weak layers and the large faults in slope coal and rock is studied. Based on the rock mass structure control theory and the limit equilibrium theory, a case study of Datang Shengli East II open-pit mine is carried out. Firstly, the monitoring data are classified. Secondly, the limit equilibrium software developed by our research group is used to quantitatively analyze the landslip mode and the mechanism of the south working pit edge. The result indicates that the stability of the current slope is declined from west to east, and the down dip weak layers can be observed. If the exposed weak layer cuts the fault, a slip of plane wedge-shape would happen, or else, there will be a slip from the sliced layer to the down dip layer with the weak layer as the bottom boundary. The potential landslip mode in the south working pit edge is the slipping of plane wedge-shape, and its bottom boundary is the down dip weak layer near the 6th coal seam floor. The F68 fault is the side boundary of the slipping solid, and the lower parts of the fault are relatively stable.