For achieving an optimized efficiency of nanowire array based solar cells, a GaAs axial pin nanowire array solar cell was designed and analyzed through simulation. The optical absorptance of GaAs nanowire was calculated using three-dimensional finite difference time domain simulations. The structure parameters of the nanowire array, such as diameter and density, were optimized, and the optimized absorptance is 87.4%. Then the electrical performance of the solar cell was analyzed by Sentaurus Device electrical simulation module. At last, the structure of the axial pin structure was also optimized according to the optical generation profile, and the final conversion efficiency of the solar cell can reach 17.6% in the optimized structure. The results indicate that the performance of the solar cell can be significantly improved through surface passivation treatment, which can reduce the surface recombination velocity of GaAs nanowires. And through decreasing the volume of the highly doped top region of the nanowire, the carrier recombination can be reduced and thus the efficiency can be enhanced. The analysis can prove some guidance for fabricating high-performance nanowire solar cells.