GaN photocathode is fully activated by employing a continuous Cs source and an alternate O source. The quantum efficiency curve of transmission-mode photocathode is tested in situ. The quantum efficiency reaches up to 13% in transmission-mode. According to the one-dimensional Schrdinger equation, the electron transmission coefficient formula of GaN vacuum electron source material is deduced. For a certain profile of photocathode surface potential barrier, the electron transmission coefficient relates to the incident electron energy, the height and the width of the surface potential. The energy band of transmission-mode negative electron affinity (NEA) GaN photocathode and the change of surface barrier in the deposit course of Cs,O are given. Using the double dipole layer surface model [CaN(Mg):Cs]:O-Cs, the NEA property formation of GaN vacuum electron source material is analyzed. The results show that the double dipole layer formed in the activation course of Cs,O is conducible to the escape of electrons, and it is the formation of double dipole layer that causes the drop of vacuum energy level of the material surface.