The study of the saturation mechanism of multialkali photocathode is of great significance for improving its electron emission performance. According to the threestep electron emission model, the electron emission process of a multi-alkali photocathode was studied by Monte-Carlo method. Simulation results were compared with the experimental data in the related literatures, which shows that when the photon energy is near the photoemission threshold of the photocathode, the obtained quantum efficiency curves and electron energy distribution curves were in good agreement with experimental data. Based on the above model, the photo-voltage effect and the space charge effect on the electron emission characteristics of multialkali photocathode were studied. Results show that the threshold laser energy density of the photocathode is about 8 μJ/cm² under the photovoltage effect, and 2.23 μJ/cm² under the space charge effect. An experimental investigation of the saturation effect of multi-alkali photocathode irradiated with 266 nm laser is conducted, and the saturation threshold laser energy density is about 2 μJ/cm², which indicates that the space charge effect is the main factor limiting its photoelectron emission capability. 