A soil arching theory and a model was put up for the thrust's transfer mechanism and the part acting on the presstressed anti-sliding pile can be computed. Aiming at the existing problems in the method for computing anti-sliding pile with prestressed anchoring cable, an improved method is proposed in this paper. According to the actual construction process and mechanical condition of the pile and the prestressed anchoring cable, the internal force of the structure should be computed in two steps. Firstly, the pile under the prestress of the anchoring cable and a part of sliding thrust will be taken into account in computation. It should be pointed out that the subgrade reaction over the slip surface behind the pile should also be taken into consideration in the calculation of the prestress. Secondly, the pile under the computed sliding thrust should be calculated and the concerted deformation of the pile and the anchoring cable must be considered. A model test on the character of the internal force of prestressed anti-sliding pile is conducted, and the pile is calculated using the improved method and the existing method. The comparison between the computation results and the test results shows that the satisfying results can be obtained by the improved method.