The spectral response performance of transmission-mode GaAs photocathode after high-temperature activation and low-temperature activation was measured by the use of on-line spectral response measurement technology, and the integral sensitivity and spectral response performance parameters of the GaAs photocathode after high-temperature activation and low-temperature activation were compared. It was found that compared to high-temperature activation, the integral sensitivity after low-temperature activation increased 13%, and both the cut-off wavelength and peak wavelength had a slight move towards long-wave. The comparison of obtained on-line measurement curves and the spectral response curve of sealed third generation imaging intensifier was further made, and it was found that there was great difference between the spectral response curves before and after sealing.