When strength reduction method is used for side slope analysis, only the intensive parameters are reduced and deformation parameters are not. The ideal elastoplastic model is adopted, and the safety factor is basically in agreement with that of the traditional limit equilibrium method. Main result is the safety factor of side slope, but non-linearity of soil is not considered actually during calculation, so it's hard to arrive at a real deformation field. Based on the deformation modulus elastoplasticity strength reduction method, research of this paper can achieve a more truthful deformation field. Considering the deficiencies in the current slip surface search techniques, this paper mainly studies how to determine the slip surface directly through the slope deformation field and stress field based on the analysis to their characteristics and the formation mechanism and characteristics of slip surface. Three methods to determine the slip surface are worked out: One is to use displacement gradient to identify the most disadvantageous slip surface; The second one is to determine the slip surface using the characteristics of FLAC calculation method and the characteristic that nodal velocity at non-slip area is near to zero and nodal velocity at slip area is very great; The last one is to use the deviation survey method in slope monitoring to track the nodal displacement in numerical analysis, and take the points with displacement catastrophe as those on the slip surface, and link those points to form a curve and finally get the critical slip surface. Analysis is made to the slip surface of clay slope with weak layer. Position of slip surface determined through these three methods is basically the same. Compared with traditional critical equilibrium method, these three methods can not only be applied to homogeneous side slope, but also be used to determine the slip surface of heterogeneous side slope. This paper makes a research on expansion of the application field of deformation modulus elastoplasticity strength reduction method.