Abstract: For the problem of spatial three-dimensional stability analysis of high slopes in engineering practice where the cut-fill interface intersects obliquely crossing with the slope strike line, an expanded three-dimensional horn-shaped failure mechanism is established based on the upper-bound limit theorem in this paper. Then, the corresponding functional equilibrium equation is formulated by highlighting the slope characteristics by introducing the mechanism parameter dip angle α of the excavation-fill interface. The sequential quadratic programming optimization algorithm is used to solve the safety factor of the upper-bound limit analysis of the strength-discounted slope. On this basis, the high slope project of Miaoling 750 kV substation in Ningxia loess hilly area and the case of the stability factor γH/c under the limit state in the relevant literature are selected as examples, and the computations are compared with the simulated values of finite element limit analysis software. The results show that the upper-bound solution and the simulated value of the safety factor F_s of the slope both decrease with the increase of the dip angle α of the dredge-fill interface. The relative error of the calculated results of the two methods is within 5.9%, and the expanded damage mechanism is basically consistent with the shear dissipation deformation mode of the numerical simulation. In the limit state, the upper-bound solution of the expanded mechanism and the numerical simulation results are very close to 1.0, and the relative error of the two methods does not exceed 8.3%. The research work provides a simple and practical method for the spatial stability analysis of high slopes.