Successful cleaning procedures for gradient-doping GaN surface have been investigated and achieved. The chemical cleaning of the gradient-doping GaN photocathode surface is carried out. The analysis of the surface after the chemical cleaning with X-ray photoelectron spectroscopy (XPS) shows that the chemical cleaning can effectively remove the organic attachments, residues in the process on the surface. Subsequent annealing of the surface under ultra-high vacuum at temperature of 710 ℃ leads to a decrease in the residual carbon, so the photocathode can obtain the ideal atom clean surface for high-performance negative electron affinity (NEA) photocathode. Finally, the photocathode active experimental results confirm that thermal annealing after chemical treatment method can effectively clean the gradient-doping GaN photocathode surface.