In order to prove the possibility and superiority of the new seismic retrofit method for an existing over through type steel arch bridge, three kinds of structural systems, such as an existing bridge, retrofitted bridges by PUFA and by concrete, were investigated using elastio-plastic and geometic nonlinear dynamic analysis. The main conclusions are followings: 1) the seismic performance of existing arch bridge is not sufficient to resist the level two earthquake described in the current design specification of highway bridges, 2) PUFA coating around of the main steel arch ribs can be used to improve the seismic performance of the existing bridge and 3) the seismic performance of the hybrid arch bridge reinforced by PUFA will be better than that of hybrid arch bridge reinforced by concretes.