Abstract: The characteristics of the development of plastic strain and the process of unstable failure for embankment of heterogeneous soils are studied with the strength reduction FEM.The unstable mechanism for embankment is analyzed according to the stress field of unstable state and formation of the critical slip surfaces.The results indicate that the strength reduction FEM is applied to stability analysis of embankments of heterogeneous soils in the theory and numeric simulation.Furthermore,the numerical non-convergence of FEM global equilibrium and displacement can be taken as a failure criterion of embankment slope.The unstable failure state can be evaluated correctly using the criterion.The relationship between the nodal displacement in the slip surface and the load increment can be investigated with reference to the plastic zone deformed from slope toe to slope crest at the same time.In addition,the stress field of unstable state in the embankment shows that it is necessary to adopt the tension-shear composite failure criterion for stability analysis of embankments to reflect the mechanism for formation of critical slip surfaces.