This paper uses a simple model, the lumped single-degree-of-freedom system on rigid mat foundation, to investigate the effects of soil-structure interaction on the dynamic response of a soil-structure system. Based on that, the key parameters affecting the natural frequency of a soil-structure system can be easily identified and be used to assess the effects of soil-structure interaction. Accordingly, it was used to simulate the dynamic response of the forced vibration tests conducted at Hualien, Taiwan. Results obtained show that the simple model can predict the field responses very satisfactorily.