Spectral response characteristics of transmission-mode GaAs photocathode after high-temperature activation, low-temperature activation and indium sealed into intensifier are measured by use of the home-made prototype of spectral response measurement instrument. The results show that spectral response curve in the whole response waveband decreases after indium seal, and long-wave responsibility is most obviously influenced. The long-wave response 800～815 nm decreases largely, cut-off wavelength and peak value wavelength move towards short-wave, peak response value and integral sensitivity decrease, and the final spectral response curve becomes flat. The calculation results of photocathode parameters reflect that surface escape probability decreases after indium seal, which indicates that indium seal leads to the variations of surface activation layers of photocathode, low-energy photoelectrons under long-wave are difficult to escape, and the long-wave response and sensitivity decrease accordingly. The influencing factors on the surface activation layers during indium seal are also analyzed.