Abstract: In order to study the deformation mechanism of soft rock composite slope，the east side compound slope with a dump site in an open-pit mine in Xinjiang is taken as the research object. This paper according to the height of the abandoned and the distance between the East and the dump to make four simulation options. Using FLAC3D for numerical simulation numerical，by analyzing the simulation results of the horizontal displacement and monitoring point displacement of the four schemes from time to time，it is found that as the dumping site is higher，the farther the dumping site is from the east side，the position of the maximum horizontal displacement of the slope is moving inside the slope; the displacement of the monitoring point fluctuates slightly at the beginning with time，decreases rapidly in the middle period，and changes smoothly in the later period; during the deformation process，due to the reduction of the restraint effect of the lower rock mass on the upper dump site and the squeeze effect of the dump field on the lower rock mass，a slope deformation chain effect is generated，which accelerates the slope deformation. The deformation mechanism of the slope is analyzed in detail from three aspects：composite，soft rock and excavation. The slip mode is obtained：pull-slip-creep，pull-slicing-slipping-creeping，splitting-cutting-creeping-cutting. Based on the creep mechanism of unstable slope，this paper proposed the mechanism of stability slope creep，it mainly includes five phases：initial deformation phase，deceleration creep phase，constant velocity creep phase，deceleration creep phase Ⅰ，and deceleration creep phase Ⅱ. The mechanism is verified by numerical simulation. The stable slope creep mechanism It has certain guiding significance for the prevention of slope slip.