Abstract：The authors consider that unstable slopes are caused by composite failure of tension and Mohr-Coulomb shear of sliding soil mass，but not caused by single shear failure. The composite failure criterion，which is applicable to rock and soil，is discussed. When the prevailing large-scale finite element programs that only the single shear model is included in their material models are used to analyze soil slope stability，there are two ways to consider shear failure of soil mass. The one is to redevelop subroutine of user-defined mechanical model，which can express the composite failure criterion of soils. The other is that the just connection between the shear failure zone and tension failure zone should be regarded as the criterion of sliding failure of soil slopes. Numerical modeling analysis is performed with seven kinds of soil slopes by using the finite element method and the fast Lagrangian finite difference method. The comparisons show that the results from the two methods considering the composite failure of tension and shear of soil mass coincide very well. Since the tension strengths of soils are lower than those of shear strengths，if the tension failure of soil mass is not taken into account，the factors of safety of slope stability are possibly 1% to10% higher than those of theoretical ones. Therefore，it is necessary to consider the tension failure of soil mass in soil slope stability analysis. Meanwhile，some laws about the shapes of composite sliding surface of the seven sorts of soil slopes are obtained；and they can be used to judge whether critical sliding surface searched by other analytical methods of slope stability is reasonable or not.
