Hypoplaticity and Nader hypoplastic model are introduced. Based on finite element stress-strain analysis of Nader hypoplastic model, slip surface stress method in the slope stability is provided. The factor of safety of the slip surface is defined as the ratio of the critical shear intensity during failure to real shear stress and the critical slip surface of slope is ascertained with pattern search method. The clay slope with a weak foundation layer and clay slope with a foundation layer including a thin weak layer under the plane strain condition are analyzed. The results are shown that the shapes of critical slip surface and the magnitude of factor of safety are obtained by this method.