The three-dimensional model of slop in numerical calculation is established. And high precision eight-node hexahedron element is used to build the finite element grid of slip layer. The slip layer material is simulated by an ideal elastic-plastic model based on the Mohr-Coulomb Criterion. Three-dimensional stress state and slide direction of the slip layer zone is obtained by numerical computation. So, based on the analysis of the three-dimensional stress state of the slip layer zone, in this article, the author defines the point safety factor as the ratio of the shear strength of the slip zone to the shear stress parallel to the sliding direction, and defines the safety factor of the landslide as the average of the zone point safety factor with its zone as weight number. The sliding mechanism is studied through zone point safety factor, and stability of landslide can be evaluated through the safety factor. Sample case shows that the calculated result is in accordance with the actual case.