The field-assisted method is proposed to increase electron capture of the GaN nanowire cathode. We deduce the photoemission or electron collection efficiencies of the field-assisted uniform-doping and exponential-doping cathodes using the diffusion-drift formula. Results show that the external electric field has a significantly better collection efficiency for uniform-doping nanowire cathodes than that of exponential-doping. In cathodes without a build-in field, quantum efficiency and collection efficiency reached a maximum at incident angles of 65 degrees and 45 degrees, and field intensity of 0.1 and 0.7 V/mu m, respectively. The external electric field also contributes to the polarization of GaN nanowires. The field-assisted GaN NWAs (nanowirearray) cathode could improve the photoemission performance and are expected to be realized in the follow-up experiment.