Abstract: In order to find the reason of sensitivity degradation in tube body of the third generation low-light-level (LLL) image intensifier, on-line quality detection was carried out by using mass spectrometer and activation process parameters were determined. The acquisition of the cathode atom clean surface, high pure cesium and oxygen source were studied. The online stability for transmission cathode photoemission of GaAs was measured. Results show that the sensitivity (1 500 A/lm) is stable in ultra-high vacuum pressure 10-9 Pa in continuous time of 500 hour. Sensitivity degradation is in connection with the vacuum loss and harmful gas pollution.