The implicit anchor cable element is adopted to simulate prestressed anchor cable in underground caverns. The equivalent additional stiffness contribution of the anchor cable attached to the rock element is discussed. Aimed at the loading process of the prestressed anchor cable, the force mechanisms of anchorage section under pre-tension and rock deformation loads are analyzed respectively. Based on internal force superposition of the anchorage section under the two loads above, a force analysis method for the prestressed anchor cable considering shear slip on the anchoring interface is proposed. Combined with the 3D nonlinear FEM, the general solving steps of force for the prestressed anchor cable are given. Then, the algorithm for the anchor cable is applied to the multistage excavations of the left bank underground caverns of Wudongde hydropower station. The results show that the anchor cable support can reinforce the rock effectively and restrict the development of damage zone. The axial force and shear stress of the anchorage section both reach the maximum at the anchor end, and decay along the depth direction. The calculated anchoring forces correspond well with the measured values, so as to verify the rationality of the algorithm.