In the seismic design of railway structures with a caisson foundation, the energy constant method has been applied to predicting the response of structures at the time of earthquake in the previous design code of foundations. In a new design code of seismic design of railway structures, dynamic analysis is adopted in view of the damage by the Hansin-Awaji earthquake in 1995. Because of larger earthquake power, bigger deformation will occur on foundation designs. In this larger deformation range, a more detailed analytical model is needed to precisely predict the response of structures. To apply nonlinear dynamic analysis, hysteresis characteristics of soil-caisson system is also required. This paper describes an outline of new seimic design method of caisson foundation and a new analytical model and the hysteresis characteristics of soil-caisson system.