Abstract: The slope movement above the mined-out area is different from the deformation and failure of general slope and overlying strata in mine area. It is an effect of both the site movements and mechanisms. This study leads to establish a new research field of mountain Environmental Engineering Geology. Hengshan slope adjacent to a power plant forms a low-angle and downdip structure mountain, and there are several coal seams beneath it. Based on analysis of the structural characteristics of low-angle rock layer, this paper deals with the mechanical properties of weak interlayers on the slope. It is considered that the weak interlayers play a leading role in the lateral movement of the slope. According to detailed field investigation and analysis of displacement data obtained, it is founded that the deformation of rock mass of the slope is mainly controlled by underground mined-out area, and collapse as well as the creeping of the rock mass on the pre-slope toward the landsurface occur. Furthermore, slope deformation process and characteristics associated with underground mine collapse were studied using geomechanical modelling and elastic-plastic finite element method. The regularity of stress change within the slope caused by mine collapsing beneath the slope is analysed. It is showed that the deformation, displacement and failure of rock formations are through the overlying strata to the landsurface of the slope, and that the weak interlayer creeping induced by mine collapsing beneath the slope leads to the slope lateral movement. The surface doming of power plant area is caused by pre-slope rock mass pushing against the basement soil. By comparing the modelling results to real deformation and displacement data and geological indications, the collapse arching effect, the effect of additional push creeping and the damaging effect of collapse arching which were induced by underground coal mining, are analysed in detail and proposed. Thereby, this paper proposed also the composite mechanism of composite movement within the composite stress field and composite boundary surfaces, associated with the collapsing of the overlying strata about the slope downdip structure mountain. The typical geological model for the lateral movement of low-angle rock formation on the slope has been put forward.