Aimed at improving the actual photoemission performance of nanowire photocathode, an axial exponential-doping GaN nanowire photocathode is proposed. Based on two-dimensional continuity equation and finite difference method, the quantum efficiency of this exponential-doping GaN nanowire photocathode is obtained. The simulation results suggest that this structure of GaN nanowire photocathode can effectively obviate the difficulty in collecting the electrons escaping from side faces because a large part of carriers will escape from top surface under the built-in electric field. Besides, it is discovered that the optimal height of nanowires is 300 nm when the doping concentration of top surface is 1 x 10(18) cm(-3) and that of back interface is 1 x 10(19) cm(-3). Then, when the nanowires are arranged as array, the optimal light angle of incidence is approximately 60A degrees by analyzing the electrons flow density of the array. By comparison of collection proportion of photoelectrons, the optimal nanowire spacing is 231 nm. This study demonstrates potential application value of exponential-doping GaN nanowire photocathode. The results can direct the preparation of this kind of photocathode.