Abstract: Compared with the common negative electron affinity photocathode, the field-assisted photocathode can significantly extend the long-wavelength threshold, so it has a broad application prospect in near-infrared light detection. In this work, the electron emission model of n-InP/p-InP/p-InGaAs field-assisted photocathode was established by using the two-dimensional continuity equations. The electron emission current was obtained by model simulation, and the external quantum efficiency was calculated. The influence of the doping concentration and thickness of the epitaxial layer on the quantum efficiency under different bias voltages was analyzed. According to the simulation results and the limitations of the preparation conditions, the optimal parameters of the cathode epitaxial structure were determined: the doping concentration and the thickness of the n-InP contact layer are 1×10¹⁹/cm³ and 0.2 μm, respectively; the doping concentration and the thickness of the p-InP emission layer are 2.2×10¹⁸/cm³ and 25 nm, respectively; the doping concentration and the thickness of the p-InGaAs absorber layer are 5×10¹⁷/cm³ and 3 μm, respectively. The widths of the electrode and the emission surface were simulated, and the optimal range of the width of the surface of the emitting unit was 5~8 μm, and the effects of the electrode width and the width of the emission surface on the quantum efficiency under different bias voltages were analyzed. Simulation results provide a theoretical basis for the structure design and applications of the field-assisted photocathode and are beneficial to the preparation of the field-assisted photocathode. The n-InP/p-InP/p-InGaAs field-assisted photocathode effectively improves the emission current efficiency, and the external quantum efficiency reaches a maximum of 17.2% at 1.55 μm under a 5 V bias at room temperature.