Using the principles of soil dynamics and structural dynamics, an analytical solution for an elastic supporting pile under horizontal vibration was formulated, based on an improved beam-on-dynamic-Winkler-foundation model and taking into consideration the change along the soil depth and the inertia phase of the fluid. The degenerated impedance on the top of the elastic supporting pile was proved to be correct. The parameters influencing the horizontal vibration characteristics of the elastic supporting pile were analysed in detail, and the influence of the elastic support on the impedance on the pile-top was discussed.