The spectral response curves of reflection-type gradient-doping GaN photocathode in the process of activation and attenuation are obtained with the spectral response measurement instrument. It’s found that the curve is constantly changing. The spectral response increases continuously in the process of activation, and the long wave response increases faster. The spectral response decreases continuously in the process of attenuation, and the long wave response decreases faster. Results show that the variation of spectral response curve is related to the escape of high energy photoelectron of photoelectric cathde. The electron energy distribution of GaN photocathode is shifted to the high energy side with increasing of the incident photon energy. Influences of cathode surface potential barrier shape change on low energy light excited electrons are even greater, which leads to different change in spectral response curve with incident light wavelength.