A model for simulating the pile-soil interaction was presented in this paper to investigate the influence of soil under pile tip on the pile impedance. In this model, the diffusion of additional stress in the soil surrounding pile tip coincides with the Boussinesq’s solution under circular loading. The soil in the main zone of the stress diffusion is treated as a fictitious soil pile. The vertical displacement at the bottom of the fictitious soil pile is restrained by bedrock. The impedance on the fictitious soil pile top is obtained via the boundary conditions at the bottom of fictitious pile and impedance transfer method. This serves as the supporting condition of the actual pile, which can be further used to solve the impedance response of the whole pile system. Results of the proposed model are compared with those obtained by Novak’s method. It is found that it can yield more accurate prediction of impedance response of pile when considering the stress diffusion in base stratum. Pile damping is proportional to the thickness of base stratum, while the stiffness is on the contrary. For the pile with smaller slenderness ratio, the influence of the stratum under pile tip is more obvious on impedance.