The photocurrent attenuation of GaAs photocathode within one hour after activation under three different vacuum pressure (5×10<sup>-9</sup>Pa, 5×10<sup>-8</sup>Pa, 5×10<sup>-7</sup>Pa) were recorded by automatically activated monitor. The results show that: the photocurrent quickly descend in the beginning and then descend linearly at a low slope; the amplitude of the quickly descending area were 10%, 14.74% and 36%separately, with the respective slope of the linear descending area were -0.00653, -0.01132and -0.02. Three samples’ gas components of H<sub>2</sub>, CH<sub>4</sub>, CO, H<sub>2</sub>O, O<sub>2</sub>, CO<sub>2</sub> etc under the same vacuum pressure (5×10<sup>-8</sup>Pa)during photocurrent attenuation were collected by quadrupole mass spectrometer. By comparing the gas components content and the attenuation law of the photocurrent, it has been found that H<sub>2</sub>O and H<sub>2</sub> had a greater impact on the stability of GaAs photocathode in the ultra-high vacuum environment and H<sub>2</sub>O was the predominant effect. This paper has important guiding significance and reference value in studying the stability of GaAs photocathode and the improvement of semiconductor photocathode process.