Abstract: A detailed surveying on the deformation of a large rockmass slope at Tongxin Village in Wushan Town of Wangxia county indicates the slope is unstable and danger. This paper presents the finding that a 1 m thick coal seam at the bottom of the slope was mined. The slope has two deformatiom phenomena: subsiding and rockmass cracking. The two phenomena continue to develop. They are closely related to the coal mining. Engineering geological comparison indicates that the subsiding and declining due to the coal mining are the main factor causing slope deforming. The UDEC was used to make the simulation of the slope deformation. The numerical results show that after the coal seam was mined, the displacement is transferred from the lower to the upper portions of the slope. The surface subsiding in the front is larger than other places, also is horizontal displacement. The deformation of the upper slope is controlled by the vertical joints, while the deformation of the lower slope is controlled by the horizontal bedding planes and the coal-mining seam. They in whole form the subsiding-declining deformation of the rock mass slope. The calculated deformation curves suggest that deformation has two phases: Sudden quick deformation and slower deformation. I If no new coal-mining activity is carried out, as the time goes on, the mountain slope itself will become stable. The isolated rock columniation in the front slope still has high movement speed after the slope system has balanced. It is instable and has to be trimmed.