A solar cell based on bottom-reflectivity-enhanced GaAs radial p-i-n junction nanowire array is proposed, and its spectral absorption and photovoltaic performance are studied by finite-different time-domain method and finite element method. The results show that replacing SiO₂ between the polymer and the substrate with MgF₂ dielectric layer with low refractive index not only significantly reduces the absorption loss of the substrate, but also significantly improves the optical absorptance of the nanowire array in the whole wavelength range. Moreover, the photoelectric conversion efficiency of the solar cell can be improved to 13.9% by optimizing the thickness of i region and the length of nanowire. This study provides a feasible way to realize low-cost and high-performance nano solar cells.