Abstract: The paper discussed the problem of the slope's deformation mechanism.It adopted 3D-σ limit elements numerical modeling and FLAC-3D numerical modeling.The purpose of 3D-σ modeling was the knowledge of slope evolvement mechanism.The purpose of FLAC-3D had two.The one of them was reciprocally validate results between the 3D-σ and FLAC-3D,and the other got deformation characters in cutting valley to understand the deformation cracking mechanism of left slope.The paper gained some conclusions by analyzing.The way that deformation mechanism of valley slope is analyzed from the point of terra stress is feasible. The unloading of slope accelerated the developing of bending and toppling deformation and drew a line of demarcation. The deformation mechanism of Jiefanggou left slope that is explained by author is rational. The unloading of slope also accelerated the developing of deep bending and toppling deformation and drew a line of demarcation too. The figure of breakage face of toppling deformation is double fold line.