Abstract: The gas components released from the glass cementation cathode module during the high temperature bake was analyzed with a mass spectrometer to resolve the problem of low GaAs photocathode emission sensitivity. The atomic level surface of GaAs electronic emission layer was obtained. The analysis result indicates that the degas temperature of the module surface is 150℃, the degas temperature of the material is 450℃, the temperature for clean surface is 580℃, and As evaporation occurs on the GaAs emission emission layer at the temperature higher than 650℃. It shows that the strict control of the clean temperature of the emission layer surface is the key to obtain high-performance sensitive cathode.