An axially exponential-doping GaAs nanowire cathode is proposed as the cathode material of photon-enhanced thermionic emission (PETE) device. Two-type theoretical models are respectively developed based on the numerical solution of multi-dimensional continuous equations with the boundary conditions. The quantum efficiency and the conversion efficiency of exponential-doping GaAs nanowire cathode based on PETE mechanism as a function of wire length, wire width, operating temperature, emissive surface recombination velocity and surface electron affinity are simulated and analyzed, respectively. Results show that the PETE devices with exponential-doping cathode can exhibit more superiority than those with uniform-doping structure by comparing the conversion efficiency, which mainly attributes to the built-in electric field induced by the axially exponential-doping structure. In addition, the optimal wire height and wire width for exponential-doping GaAs nanowire cathode are about 6 inn and 300 nm, respectively. Moreover, the emissive surface recombination velocity has an obvious inhibition on the quantum efficiency and conversion efficiency of GaAs nanowire PETE devices. The simulations would provide valuable theoretical guidance to achieve high-performance nano-based PETE devices.