Abstract: Two-stage proton acceleration scheme by laser radiation pressure is investigated. When an ultra-intense laser interacts with a solid foil with backside underdense plasma, the laser ponderomotive force pushes the electrons in the foil forward and an electron-ion double layer is produced. As the laser continuously pushes the double layer into the backside plasma, the double layer acts like an intense moving electric field propagating through the underdense plasma with an almost steady speed. The protons at the backside plasma can be reflected and trapped by this moving field and be accelerated to high energies. The proposed scheme is studied by both two-dimensional particle-in-cell simulation and theoretical analysis. The proton energy spectrum shows a good profile with maximum energy above 20 GeV even after a long time. Proton acceleration with this scheme is more effective than that by the simple radiation pressure acceleration. The simulation results agree well with that of the theoretical analysis.