Metal organic chemistry vaporation deposition (MOCVD) epitaxial p-type GaN layer is used as emission material of GaN photocathode. By using on-line measurement of photocurrent in activation process,the influences of Cs activation,Cs/O alternate activation and high-low temperature two-step activation techniques on photoemission performance of GaN photocathode are investigated. The experimental results show that GaN photocathode can obtain about 20％ quantum efficiency only by Cs activation,and the quantum efficiency can be increased slightly by two or three Cs/O cycles. But the high-low two-step activation technique can not increase the quantum efficiency. The experimental phenomena are explained by the dipole surface model.