The degradation of activated GaAs vacuum electron source as a function of time has been investigated by using X-ray photoelectron spectroscopy (XPS).We found that the cathode surface element content changes with time and the significant decrease in sensitivity of electron source is mainly due to the change of dipole direction caused by the adsorption of harmful gases on the cathode surface.Based on the above results,we deduced the degradation model of GaAs electron source through analyzing the adsorption process of harmful gases on the surface in vacuum system.The model reveals the exponential degradation rule of GaAs electron sources and the inverse relationship between lifetime and pressure.The theoretical results are in full agreement with the experimental fact.