Abstract: The change of GaAs photocathode surface composition was analyzed by XPS before and after the photocathode was annealed and activated, combined the pressure curve of CO2, H2O, O, As, Cs obtained through four mass spectrometer while the photocathode was annealing, the difference of the two annealing was compared and discussed. It was Proposed that for the purpose of the second heating was not only cleaning photocathode surface, it was more important to procure first activated in a photocathode formed on the surface of the dipole layer is transformed to the more stable structure at high temperatures, Cs2O dipoles reacted with GaAs and generated on the photocathode surface of the stable existence of the GaAs-O-Cs dipoles, formed dipoles layer mainly by bonding strong GaAs(Zn)--Cs+, GaAs-O-Cs dipoles and Cs2O dipoles which was attached to the photocathode surface by the Van der Waals force. Based on this conclusion, The differences of light current variation between the activation of the photocathode was explained. It was of great significance to understand the activation of the photocathode and photoelectric launch model.