GaAs nanowire cathodes with exponential doping and graded Al composition structures are proposed for photon-enhanced thermionic emission (PETE) devices. The conversion efficiency models with these two nanowire cathodes are deduced on the basis of one-dimensional continuity equations. The conversion efficiency as a function of wire length, wire width, Al composition distribution, cathode temperature, emissive surface and back interface recombination velocity are also simulated, respectively. Results show that exponential doping and graded Al composition cathode structures can obviously improve the conversion efficiency of devices through introducing a built-in electric field along the growth direction of nanowire. Besides, the optimum wire length and wire width range are 300-340nm and 5.9-6.4m, respectively. Moreover, wider Al composition range is beneficial to achieve higher conversion efficiency. These simulations provide an interesting attempt to explore the working mechanism of GaAs nano-based PETE devices and are expected to be verified by the experimental results in the future.