In order to get the quantitative relationship between Cs adsorption and cathode electron affinity variation, GaN photocathode was activated with Cs using the experimental system for activating and evaluating NEA photocathode. According to the theory of semiconductor photoemission and the double dipole layer model, the electron affinity changes with the Cs coverage degree, the function relation between electron affinity and Cs coverage degree were gotten. Cs adsorption mechanism was analyzed, the relationship between the Cs adsorption course and the decrease of effective electron affinity was gotten. The experiment results show: during the course of activation with Cs for negative electron affinity GaN photocathode material, the photocurrent increases from the background value to the maximum according to Cs coverage degree. The position of low kinetic energy cutoff in the electron energy distribution curve is decided by Cs coverage degree during the activation process. When Cs coverage degree varies from 0, 1/2, 2/3 to 1 monolayer, the position of low kinetic energy cutoff shifts to the left in turn. The position of low kinetic energy cutoff will shift to the left about 3eV when Cs coverage degree increases from 0 to 1 monolayer. The reason of the left shifting is the quantity increase of the dipole ［GaN(Mg):Cs］ that help the electrons to escape into the vacuum. The quantity increase of the dipole results in the decrease of vacuum energy level of the surface.