The massless base model and viscous boundary base model are still the most widely applied approximate methods for easiness though many approaches have been proposed for the seismic response analysis of soil-structure system. But there are some problems in the application of various of approximate dynamic unbounded soil model. In this paper

detailed formula for the soil-structure dynamic interaction analysis is derived; the concept and the application criteria of mass-less base model and viscous boundary base model are clarified; and a simplified soil model for dynamic soil-structure interaction analysis is suggested.