Due to the long free section and the lack of soil resistance on the free surface of the foundation pile lo? cated on the steep slope，the deformation has an obvious P-Δ amplification effect. In this paper，introducing the en? ergy variational principle，considering the complexity of the pile bearing and pile-soil interaction in the steep slope section，and assuming that the soil displacement is a nonlinear attenuation mode along the radial direction of the foundation pile，the total energy control differential equation of the three-dimensional foundation pile-soil system in the steep slope section is established. On this basis，combined with different boundary conditions and the coordina? tion conditions of pile-soil displacement，the semi-analytical energy method solution for the deformation of the foun? dation piles on steep slopes under complex bearing conditions is obtained based on the finite difference method，and the solution can fully consider the P-Δ effect of the pile deformation in the steep slope section under various loads. Then，the indoor model loading verification test of the single pile under the axial and lateral load on the steep slope section was carried out，and the theoretical calculation value was compared with the measured value. The compari? son results show that the theoretical solution in this paper is in good agreement with the test results. Finally，a quanti? tative analysis of the influencing factors of the P-Δ amplification effect of foundation piles on steep slopes is carried out. The results show that the P-Δ amplification effect of bridge foundation piles on steep slopes increases signifi? cantly with the axial and lateral load ratio and the free length of the pile top but decreases with the increase of soil stiffness. At the same time，the slope gradient has a significant increase influence on the P-Δ effect.