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In this paper, the "three-step model" of photoemission mechanism for gallium nitride (GaN) photocathode is discussed. The first step is the absorption of incidence light. The incidence photon energy is absorbed by the electrons in the valence band, and this establishes the foundation that the electrons in the valence band are stimulated to the conduction band. The second step is the transportation of photoelectron, the excitated electrons in the conduction band will move from bulk to surface by diffusing or drifting.The third step of cathode photoemission is that the photoelectron passes through the surface potential.