Abstract: The determination of the seismic effects for an underground structure often depends on the empirical approach or the one-dimensional equivalent linearization method. However, because of the strong nonlinear feature of the soil, the site always plastically deforms after the seismic excitation, which will bring up additional loading effects for the underground structure buried in the soil. 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 were analyzed, using the results of 1-g 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 on the tunnel obviously varied. It leaded to the increase of the bending moment and the structural deformation was more concentrated at the connection regions. The structure also had some vertical shrinking and horizontal expanding deformation responses. A region of the structure contacted hardly with soil, which resulted in the earth pressure concentration at the region. Therefore, the computation under the unbalance loading condition is suggested to be added in the design of an important underground structure, which could help to avoid the regional damage of the structure.