GaN samples 1-3 are cleaned by a 2:2:1 solution of sulfuric acid(98%) to hydrogen peroxide(30%) to de-ionized water;hydrochloric acid(37%);or a 4:1 solution of sulfuric acid(98%) to hydrogen peroxide(30%).The samples are activated by Cs/O after the same annealing process.X-ray photoelectron spectroscopy after the different ways of wet chemical cleaning shows:sample 1 has the largest proportion of Ga,N,and O among the three samples,while its C content is the lowest.After activation the quantum efficiency curves show sample 1 has the best photocathode performance.We think the wet chemical cleaning method is a process which will mainly remove C contamination. GaN samples 1-3 are cleaned by a 2: 2 solution of sulfuric acid (98%) to hydrogen peroxide (30%) to de-ionized water; hydrochloric acid (37%); or a 4: 1 solution of sulfuric The samples are activated by Cs / O after the same annealing process. X-ray photoelectron spectroscopy after the different ways of wet chemical cleaning shows: sample 1 has the largest proportion of Ga (98%) to hydrogen peroxide , N, and O among the three samples, while its C content is the lowest. After activation of the quantum efficiency curves show sample 1 has the best photocathode performance. We think the wet chemical cleaning method is a process which will mainly remove C contamination.