Four different p-type doping GaN photocathodes are activated by Cs/O, and the quantum efficiency (QE) curves are obtained. According to the QE equation, the curves are fitted. Both the QE curves and the fitting results show that the optimal p-type doping concentration is at 1017 cm-3. The electron diffusion length and surface-electron escape probability can be balanced well at 1017 cm-3. To a certain degree, thick emission layer is conducive to improving the QE, which is more obvious with the long wavelength.