The photocurrent attenuation of GaAs photocathode within one hour after activation under three different vacuum pressure (5×10-9Pa, 5×10-8Pa, 5×10-7Pa) 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 H2, CH4, CO, H2O, O2, CO2 etc under the same vacuum pressure (5×10-8Pa)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 H2O and H2 had a greater impact on the stability of GaAs photocathode in the ultra-high vacuum environment and H2O 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.