Photocathode has received increasing attention in scientific devices that require high-quality electron beams. A theoretical photoemission model for enhancing the performance of In0.5Ga0.5N tilted nanowire array photocathodes with negative electron affinity surface is proposed. Finite Element and finite difference methods are employed in simulation experiments to study the In0.5Ga0.5N tilted nanowire array photocathode. The results demonstrate that the photoelectric conversion performance of the In0.5Ga0.5N tilted nanowire array photocathode can be significantly enhanced through the combined impact of oblique incident light and an additional electric field. Specifically, when θ = 20°, β = −40°, and Eout = 4 V/μm, the total quantum efficiency and collection efficiency are 2.79 times and 4.27 times higher than compared to vertical nanowires without external assistance, respectively.