Mechanism about the change of GaAs phtotocathode's surface barrier during Cs activated process was studied. Ionized Cs and p type doping impurity(Be) form a dipole that decreases the vacuum level of GaAs. Generally, Cs activated GaAs photocathode could achieve zero electron affinity state. The quantum yield formula of reflection-mode photocathode has been solved from the 1-dimension continuity equations and the escape probability formula has been solved from the Schrodinger equation. It was found from the formula that the quantum efficiency of Cs activated GaAs photocathode is directly proportional to the electric field intensity of Be- -Cs+ dipole. A Cs activation experiment was carried out, the experiment process tallies with the theory mentioned above and the integral sensitivity of Cs activated GaAs photocathode is 453 microA x lm(-1), which could be inferred as a zero electron affinity state.