In this study, Expanded Polystrene (EPS) geofoam is used as seismic buffer to mitigate shock damage of tunnel by earthquake. Four types of EPS layer with different elastic modulus are selected to investigate their isolation effect. Four kinds of thickness are also selected to study the effect of thickness. All tunnels with different EPS layers are compared with each other and with the tunnel without EPS layer. Detailed simulation technology is described in this paper, including the infinite element boundary, initial ground stress balance and material models. The numerical analysis show that the axial strain and section force of lining are greatly reduced compared with the tunnel without EPS. With the decreasing elastic modulus and increasing EPS layer thickness, the mitigation effect is more obvious. And the isolation effect of reducing modulus is greater than that of increasing thickness. So modulus of EPS layer is a more important factor for mitigating dynamic damage.