In modern low-light-level night vision devices, the negative-electron-affinity GaAs photocathode, as the photosensitive core component, is usually prepared by alternating Cs/O activation on the cleaned GaAs surface. Meanwhile, the stability of GaAs photocathode is directly affected by the quality of Cs/O activation process and the residual gases in the vacuum environment. In order to improve the stability of GaAs photocathodes after activation and prolong the operating lifetime of the low-light-level night vision devices, experimental researches were executed from the perspectives of Cs/O_{activation method and decay characteristic with the aid of the self-developed ultra-high vacuum photocathode preparation and multi-information on-line measurement and control system. Two different Cs-O activation methods, namely the traditional 'yo-yo' method and the improved 'yo-yo' activation method were performed on the p-type GaAs (100) substrates grown by the vertical gradient freeze method. During the traditional 'yo-yo' activation process, the O source was introduced when the photocurrent dropped to 85% of the previous peak and the Cs source was kept continuously with a slight overdose, and then the O source was closed when the photocurrent reached a new peak. While in the process of the improved 'yo-yo' activation, the Cs source was maintained continuously and the O source was introduced when the photocurrent dropped to the minimum with the complete Cs overdose in the Cs/O alternate activation cycles, and the O source was also closed when the photocurrent reached a new peak. The Cs/O activation results show that the improved 'yo-yo' activation method with less alternate activation cycles can obtain a higher photocurrent peak and a higher spectral sensitivity than the traditional one with more alternate activation cycles. In addition, the photocurrent decay results under the long-time illumination of 633 nm demonstrate that the GaAs photocathode with the improved 'yo-yo' activation method can achieve a longer operating lifetime and a better stability than the traditional one. After 18 hours of decay under continuous illumination, the GaAs cathode sample using the improved 'yo-yo' activation method exhibited a smaller drop of spectral sensitivity, especially in the near-infrared waveband. obvious. At 800 nm, the sensitivity of the cathode sample using the improved 'yo-yo' activation method is decreased by 37%, while that of the cathode sample using the traditional one is decreased by 63%. Furthermore, by measuring the changes of components and partial pressure of residual gases in the activation chamber with the quadrupole mass spectrometry, and through fitting the photocurrent decay data based on the decay model related to the vacuum pressure and the partial pressure of residual gases, the weight factors of influence of different residual gases on the decay of photocathode performance were obtained. The fitting results show that the water vapor and carbon dioxide have the greatest impact, followed by methane and carbon monoxide, while hydrogen has almost no impact, and other hydrocarbon organic molecules also have the negative impact. By comparison of weight factors of residual gas components, it is found that the improved 'yo-yo' activation method has better immunity to the degradation of GaAs photocathode performance caused by the adsorption of residual gas molecules in the vacuum chamber than the traditional one. In order to verify the improvement effect of the improved 'yo-yo' activation method on the immunity of oxygen molecules, the photocurrent decay cases of GaAs photocathode samples activated by the two different activation methods, were tested under the illumination of 633 nm red light by introducing oxygen with different partial pressures into the activation chamber. The results show that the decay rate of photocurrent increases with the increase of oxygen partial pressure. Whereas, the photocurrent decay rate of GaAs photocathode sample with the improved 'yo-yo' activation method is significantly lower than that of GaAs photocathode sample with the traditional 'yo-yo' activation method. When the partial pressure of oxygen is 6×10^-10 Pa, the photocurrent decay rate of the improved 'yo-yo' activation method is reduced by 75% compared with that of the traditional one, and when the oxygen partial pressure is 1.2×10^-9 Pa, the photocurrent decay rate of the improved 'yo-yo' activation method is reduced by 54% compared with that of the traditional one. In general, the improved 'yo-yo' activation method can obviously delay the performance degradation rate of GaAs photocathode caused by the adsorption of oxygen-containing gas molecules on the surface, which will help to improve the stability of GaAs photocathode in low-light-level night vision devices.}