GaAs photocathodes are widely used in low-light night vision owing to their high quantum efficiency and adjustable spectra. In particular, they are distinguished from multi-alkali photocathodes based on their high integration sensitivity. The negative electron affinity of GaAs photocathodes is determined through Cs, and O activation is achieved. However, after activation, the maintenance of negative electron affinity is affected by many factors, such as the activation source, activation method, and gas atmosphere. To explore the factors that affect the stability of GaAs photocathodes in ultra-high vacuum systems, an activation and stability experiment was performed with a GaAs photocathode. The activation photocurrent curve and gas composition in a chamber were monitored. The experimental results show that in a high-vacuum system with vacuum degree less than 1×10.6 Pa, the stability of the GaAs photocathode was not directly affected by the degree of vacuum but by the gas composition in the chamber. Among these, H2O had the greatest impact on stability. The increase in the H2O partial pressure in the vacuum system rapidly destroyed the Cs and O activation layers of the GaAs photocathode and dramatically reduced the photoemission.