We numerically studied the effect of the geometric structure and Al component on the optical capture performance of gradient Al component Al_xGa_{1  −  x}N photocathodes. The effects of geometric parameters, such as base radius (R), wire-to-wire spacing, cone rate, and angle of incident light, on the optical response were systematically studied based on the finite element method. In the radial direction, we study the optical response of rectangular periodic structure and hexagonal periodic structure. Simulation results show that pencil nanostructure can achieve omnidirectional and broadband light absorption of Al_xGa_{1  −  x}N nanowires with hexagonal periodic structure. In addition, we used the Spicer three-step emission model to establish the photoemission efficiency of the Al_xGa_{1  −  x}N nanostructure. As a result, the photocathode achieves optimal quantum efficiency when the Al component is in the range of 0 to 0.75 and sublayer thickness of 240, 180, 120, and 60 nm.