Abstract: In order to improve the stability of GaAs photocathodes after Cs-O activation and prolong the lifetime of image tubes, Cs-O activation was studied to find appropriate solutions. Combined changing the excess amount of Cs in activation with online monitoring of photocurrent in UHV, the factors influencing the stability of GaAs photocathode were investigated. Three groups of experiments were carried out, and same five kinds of Cs excess proportion were included in each group. After the photocurrent decreased to 90%, 70%, 50%, 30% and 10% of the Cs peak value, oxygen was introduced for alternate activation. It was found that the stability of photocathodes with Cs excess of 90%, 70% and 50% was better and the stability of 30% or 10% was worse by online monitoring of photocurrent during 30 min in UHV of less than 110-8 Pa. The results indicate that with more Cs amount in Cs-O activation the construction of surface barrier is more integrated and the stability of photocathode is better. These results are important for improving the stability of GaAs photocathodes and prolonging the lifetime of image tubes.