Photocurrent of GaAs photocathode activated with Cs and O was tested by auto-activation monitor, the fitting curves of photocurrent showed that the photocurrent of the photocathode after the first activation declines exponentially, and then declines linearly with very small slope |k1|; the photocurrent after the second activation rises exponentially, and then declines linearly with a slope|k2| which is a bit larger than |k1|.Based on the mechanism difference between twice annealing of the photocathode, the degeneration behavior of the photocathode was analyzed by three-dipoles model and XPS test after the first activation and succedent thermal cleaning. It is indicated that Cs2O dipoles on the surface are saturated after the photocathode was activated for the first time, the remained Cs and Cs2O in the ultra-high vacuum chamber which deposited on the photocathode surface will prevent the emission of photoelectrons. The photocathode surface with Cs and O reconstructed when it was annealing for the second time, a lot of Cs2O dipoles changed into more stable GaAs-O-Cs dipoles, and this phenomenon would happened immediately as soon as the photocathode was activating for the second time. After the residual Cs and Cs2O dipoles depleted, the neutral gas CO2, H2O, O2, damaging the surface dipoles layer, are the main factors resulted in the decline of photocurrent. Due to the instable Cs2O dipoles on the surface of photocathode have greater chances of converting into stable GaAs-O-Cs dipoles when photocathode was activated for the first time, the photocurrent declines more slowly compared with the second activation. The discussion for the phenomenon is of great significance for exploring the photoemission mechanism of Ⅲ-Ⅴ semiconductors.