Abstract: Regarding the typical engineering as the prototype, we utilize the FLAC3D finite difference software to build the model of a three-dimensional slope, and analyze the impact on slope dynamic response characteristics and dynamic instability mechanism which are influenced by stepped slope platform width. The results show that the slope platform can effectively improve the slope stability under seismic dynamic conditions; the slope with greater platform width will be more stable; and the PGA amplification factor decreases with the increase of platform width; moreover, the first step dynamic response of a slope on the top of the hill is more outstanding than the second step toe with the combination of spectrum analysis. The slope’s rock and soil shear strain increment and the displacement response is also reduced. Plastic zone of slope under earthquake develops from the toe area to inner slope increasing with the earthquake duration; at the same time hill will be tensile deformation; slope and shallow surface layer will be tensile shear deformation, and certain parts of the slope will present a shear deformation. Displacement curves of slope monitoring points show that slope toe displacement is upward under earthquake; and the cut deformatiobn will occur; the slope slope toe and the point of gradient change become weak parts; so it should be protected tightly. The results have a guiding significance to the seismic design of earth slopes.

Key words: grade slope, platform width, dynamic response, numerical calculation, plastic zone