High-energy proton beams have broad application prospects in medical imaging, tumor therapy and nuclear fusion physics. Laser plasma acceleration is a new particle acceleration method with great potential because its acceleration gradient can reach $10^{3}\sim10^{6}$ times that of traditional acceleration method, so it can theoretically accelerate electrons and ions to high energies in the scale of a few centimeters to a few meters. Radiation pressure acceleration (RPA) is considered to be the most promising mechanism of high energy proton acceleration in laser plasma acceleration, but the Rayleigh-Taylor instability (RTI) inherent in the process of radiation pressure acceleration will cause transverse density modulation on the target surface, resulting in the premature termination of the proton acceleration process and the failure to obtain high energy proton beams. In order to obtain high-energy proton beams, an acceleration scheme combining radiation pressure acceleration with laser wakefield is proposed. In this scheme, a high-energy proton beam with peak energy of 22.2GeV, cut-off energy of 36.4GeV and charge of 0.67nC is obtained by adding a uniform density plasma channel at the back end of the thin target with critical density, the cut-off energy of the high energy proton can be increased by an order of magnitude compared with the proton only in the radiation pressure acceleration process. The results confirm that in a uniform-density plasma channel connected behind a thin target, the laser wakefield can capture protons pre-accelerated by the radiation pressure process and maintain the acceleration for a long period of time, finally obtain high-energy protons. The acceleration of protons in plasma channels with different uniform densities is also investigated in this work, and it is found that the higher the density, the higher the peak energy, cut-off energy and charge of the accelerated protons are. The combined acceleration scheme is instructive for the generation and application of high-energy proton beams.