Photocathode materials with the lower surface doping density need a longer time to raise photocurrent in the first Cesium activation process when the system vacuum level is not high enough, which can be found from the photocurrent curves during the activation of negative-electron-affinity (NEA) GaAs photocathodes. At the same time, with the enhancement of system vacuum level, these differences in time will become unobvious. It is indicated that the adsorption efficiency of Cesium on cathode surface has the direct relationships with the surface doping density and system vacuum level. In order to analyze these relationships quantitively, in this paper, a mathematical model of the adsorption efficiency of Cesium on cathode surface is established according to the experimental data. The simulation results by the model are in good accordance with the experimental phenomenon. This study is of very important value and significance for the further investigation of structure design and preparation techniques for varying doping GaAs photocathode materials.