Abstract: The improvement of vibration characteristics of caisson foundation due to the piles added at the bottom is studied. Firstly, dynamic equilibriums for lateral vibration of caisson-pile composite foundation are created through the displacement coordination and internal force equilibrium s at the interface between the two parts of the caisson and piles. Vibration equilibriums for the part of caisson are created by external force balance with the utilization of dynamic Winkler model, and vibration characteristics of the part of piles are defined by the existing literatures. Secondly, the coefficients of dynamic Winkler model in vibration of caisson are studied, and improved expressions are derived. Finally, taking a composite foundation in layered soils as an example, the improving effect of piles under the caisson is observed. This example shows that the piles under the caisson can improve the dynamic performance of the foundation significantly.