Abstract: The current strength reduction methods for in slope analysis generally only reduce strength parameters and do nto reduce deformation parameters. Using the ideal elasto-plastic model, the calculated safety factor is almost the same as that by the traditional limit equilibrium methods. Those methods only give the safety factor of slopes. However, they cannot reflect the nonlinear properties of soil, so they cannot give real deformation field. Based on the variable modulus elastoplastic strength reduction method, the real deformation field can be obtained. Aiming at the deficiency of the current searching methods for slip surface and considering the features of deformation and stress of slopes as well as formation mechanism and characteristics of slip surface, how to determine the slip surface by deformation field and stress field of slope is studied. Three methods are discussed: (1) the most unfavorable sliding surface is determined by displacement gradient; (2) according to the computational features of FLAC, the characteristics are adopted that the nodal velocity of stable region is zero while the nodal velocity of slip region is very large to determine the slip surface; (3) referring to inclinometer concept in slope monitoring, the nodal displacement is tracked, and the nodes with displacement mutation are taken as the points of slip surface, then the critical slip surface is the fitted curve of those points. A clay slope with weak layer is analyzed, and the slip surfaces by the above three methods are nearly the same. Compared to the traditional limit equilibrium methods, these three methods can be applied in