As the pile foundations in slope areas near rivers are increasingly adopted，the problem of pile-soil horizontal coupling vibration of piles in slope foundations has also received mounting attention. Based on the existing theory of horizontal dynamic response of foundation piles in level ground，this paper proposes an analytical solution for the horizontal vibration response of foundation piles in sloped ground considering the slope effect. Firstly，by means of differential transformation，Helmholtz decomposition and separation of variables method，the threedimensional wave equation of soil is decoupled，and the continuous condition of the pile-soil boundary is introduced to solve the horizontal dynamic resistance of the surrounding soil of the foundation pile in the level ground. On this basis，the horizontal dynamic resistance of the soil around the foundation pile in the sloped ground can then be de? duced by introducing a reduction factor to consider the weakening effect of the sloped ground on the soil resistance on the side of the free surface and ignore the horizontal dynamic resistance provided by the shallow soils within a certain depth；in addition，governing equations for the horizontal vibration of the free section and the submerged section of the foundation pile in the sloped ground is deduced based on Euler beam model；the analytical solutions of the hori? zontal vibration response of the foundation pile can then be obtained by the transfer matrix method，including the dy? namic resistance of the foundation pile and the internal force and deformation of the pile shaft. The proposed solu? tions are verified by comparing with those existing analytical solutions of dynamic cases in the level ground and nu? merical simulations for static cases in the sloped ground in terms of internal force and deformation of the founda? tion pile.