NEA GaN-based photocathode has important application prospects in the vacuum electron source and ultraviolet detection due to its direct wide bandgap, high quantum efficiency, and robust characteristics. In the recent 20 years, research on the NEA GaN photocathode has been extensively explored. Herein, we provide a comprehensive review on the basic theory, structure design, fabrication, and performance characterization of the NEA GaN photocathode from the following aspects: (a) the three-step model, quantum efficiency equation, and surface model of the NEA GaN photocathode are introduced and discussed; (b) the basic structure, p-type doping concentration, thickness of the GaN emission layer, and development structure are reviewed and discussed; (c) the GaN photocathode's material growth, surface cleaning, and activation are summarized and discussed; (d) the quantum efficiency, stability, and recovery characteristics of the NEA GaN photocathode are reviewed and discussed. For each part, the underlying physical mechanisms are shown in detail and the existing research results are summarized. It can be expected that this review can be beneficial for the development of the NEA GaN photocathode from both the theoretical and experimental points of view and promotes practical applications in the vacuum electron source and UV detection.