The spectral response and quantum yield curve of reflection mode GaN photocathode just after Cs, O activation and Cs reactivation was achieved by using the online multi-information measurement and evaluation system. Also the attenuation in photocurrent under the radiation of 300 nm light is measured every hour.The result indicates that GaN photcathdoe are much more stable than narrow band material. The photocurrent peak increased by 16.8% after Cs reactivation which demonstrates the reason of the QE attenuation is the Cs desorption on the Cs, O adlayer of surface. This can be explained by a double dipole layer model :Cs-O whose stability determines the stability of GaN photocathode.