Abstract: The photocathode surface barrier formation process of the GaAs photocathode was simulated according to the variation of the photocurrent while the photocathode was activating and in situ tests of spectra response, the two-dipole model was amended to establish a three dipole model. It was considered from the new model that the photocathode surface barrier formed by three kind of dipole layers, the first dipole layer was composed of GaAs(Zn)-Cs+ dipole, the second dipole layer was composed of Cs2O dipole and the third dipole layer was composed of GaAs-O-Cs dipole, the second and third dipole layer embedded in the first dipole layer. The barrier potential distribution which was piecewise uniform was established according to tunnel effect and results of quantum efficiency tests, it was calculated that the width of the barrier is 1.65 nm, and the effective electron affinity energy is -0.44 eV. The establishment of the new model is of great significance to further understand the photocathode surface emission mechanism.