The photocurrent curves during either Cs or Cs/O activation of GaN photocathode were tested by using dedicated experimental system for activation and evaluation of negative electron affinity （NEA） photocathode. Aiming at explaining the formation of NEA property of GaN photocathode and according to the rule of photocurrent change during activation period and the surface model of a fully activated photocathode，the activation mechanism for NEA GaN photocathode was studied. The experiment results show that：the obvious NEA property is be induced in GaN photocathode mainly due to the activation by Cs. The increase extent of photocurrent is not large after introducing O during Cs/O activation process for GaN photocathode. The NEA property formation reasons of GaN photocathode after being activated successfully can be well explained using the double dipole layer model ［GaN（Mg）：Cs］：O—Cs.