An effective seismic retrofit for a sidewall of a cut and cover tunnel has not been established yet. We develop a new method of seismic retrofit for the cut and cover tunnel. We call this “polymer isolation method.” In this method, we insert a thin wall made of polymer material in which the wall is called “isolation wall.” The effectiveness of the proposed method is demonstrated through some numerical simulations. It is found that the proposed method depends on thickness of the covered soil over the tunnel and ratio of stiffness between soil and structure. Finally, a simple chart is proposed to represent an applicability of the polymer isolation method for convenience to design seismic retrofit of a cut and cover tunnel.