Abstract: A four-story steel frame model on the soil-trough was prepared with the scale of 1/4,and excited to acquire the acceleration responses in different working conditions. And then the structure was fixed to the rigid ground and re-excited with the same seismic wave under the same working conditions,and the acceleration responses of the structure were acquired again. According to the dynamic equilibrium equation,the bottom shear force of the structure was calculated and compared between the flexible foundation structure and the rigid foundation structure. The results show that the relative stiffness ratio between superstructure and ground soil is a key factor for soil-structure interaction. As the relative stiffness grows,the bottom shear force changes significantly.