By making use of the self-developed ultrahigh vacuum activation system and surface analysis system, the validity of GaN (0001) chemical cleaning formula is confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The heating cleaning technique to clean atom surface is proved to be correct by activated result. The activation technique combining Cs single activation with follow-up Cs /O alternative activation is established by integrating light current changing curve in the activation process. GaN photocathode whose surface achieves negative electron affinity (NEA) is prepared successfully. After the activation, the spectral response is measured by the illumination of optical fiber. The whole preparation technique is proved to be correct true according to quantum efficiency curve obtained by calculation. The technique process of preparing NEA GaN photocathode is established through a series of experments.