To obtain a seismic design method for underground structures, which has simple model with higher accuracy and can better reflect the soil-structure interaction, this paper presents a modified response displacement method. The two-parameter foundation model was adopted to simplify the soil, which is closer to the soil properties than the traditional single-parameter foundation model. Based on the force and deformation properties, the two-parameter computational model was simplified, and the modified response displacement method was obtained, which considers soil-structure interaction. Compared with the traditional model, the proposed model only modifies the foundation bed coefficient of normal spring, making it still a simple computational model. In addition, a numerical model was established to verify the correctness of the modified method and compared with the traditional response displacement method based on the results of dynamic time-history calculation. Results show that the error of the modified method could be reduced by half compared with the traditional response displacement method. Therefore, the proposed method has simple model and high computational accuracy, which can be applied in the seismic design of underground structures.