GaN ultraviolet (UV) photocathode is a type of photoemission material with negative electron affinity (NEA) surface.The photocathode has high quantum efficiency,low dark current,good stability and many other virtues,therefore it has become a new-type UV detection material in recent years.Using atoms absorption technique in ultra-high vacuum system,experiments of high-low temperature two-step preparation are made on p-type MOCVD expitaxial GaN.The two-step preparation includes four steps,which are high temperature cleaning,Cs/O activation,low temperature cleaning and Cs/O activation.The experimental results show that GaN photocathode with about 20% quantum efficiency is obtained after high temperature cleaning and Cs/O activation.Photoemission current is still observed after low temperature cleaning.After subsequent Cs/O activation the photocurrent of photocathode can be restored,but the value is only close to the level after high temperature activation.This indicates that GaN photocathode material only needs single-step activation to be prepared.The differences of two-step preparation effects between GaN and GaAs photocathode material are compared,and preparation mechanisms of GaN photocathode is also discussed.