Aimming at the decay tendency of reflection-mode negative electron affinity (NEA) GaN photocathode and the different decay speeds of quantum efficiency corresponding to the different wave bands, and referring to the decay tendency of quantum efficiency curve provided by foreign authors for reflection-mode NEA GaN photocathode, the quantum efficiency decay mechanism for reflection-mode NEA GaN photocathode was studied. The surface model ［GaN (Mg) : Cs］: O-Cs for GaN photocathode after being activated with cesium and oxygen was used. And the change of surface barrier in the decay course of quantum efficiency was considered. The reduction of the effective dipole quantity is the basic reason causing quantum efficiency reduction. And it is the change of surface I, II barrier shape that causes the difference of dropping speeds of quantum efficiencies corresponding to different wave bands.