In the seismic design of well foundations, it is important to understand the behavior of those with large deformation. Because of the structural complexity of steel-pipe-sheet-pile foundation (SPSPF), there have been no analytical models which can calculate the response of SPSPF at high precision. In this study, the interaction among the members of foundation is evaluated and then, a new structure-analysis method has been developed to calculate the response of SPSPF exactly under the condition of large deformation. Moreover, a rational design method has been developed for diaphragm-well foundations to reflect the effect of input-seismic-motion loss.