Abstract: Because of strong nonlinear features of soils, the site always plastically deforms after a seismic excitation, which will have additional loading effects for underground structures buried in soil. But the effects are not taken into account in the current seismic design methods, which may bring up safety risk for underground structures. In the paper, the free-field and non-free-field site's plastic deformation and their influence on the seismic responses of a tunnel structure are analyzed, using the results of 1g shaking table model tests of a precast horseshoe segmental tunnel and the corresponding free-field site. Results show that when the site plastically deformed, the earth pressure distribution around the tunnel obviously varied. It leading to the increase of the bending moment and the structural deformation is more concentrated at the connection regions. The structure also has some vertical shrinking and horizontal expanding deformation responses. A region of the structure has a hard connection with the soil, which results in earth pressure concentration at the region. Therefore, the computation under the unbalanced earth and water static pressure condition is suggested to be added in the design of an important underground structure, which could help to cover the internal force response might be have.