We numerically analyzed the influence of geometric structures of gradient Al component Al_xGa_{1  −  x}N nanowire on their light-trapping properties, ranging from nanopillars, inverted conical frustums, to inverted hexagonal frustums. COMSOL^® Multiphysics package based on the finite element method is used to systematically study the effects of geometric parameters such as base radii (R), pillar height (H), period (P), and angle of incident light on the optical absorption. The simulation results show that compared with the other nanostructure counterparts, the inverted hexagonal frustum can effectively couple photons into the nanoarrays to achieve wide spectrum and effective optical absorption for Al_xGa_{1  −  x}N nanowire-based UV photocathode. The inverted hexagon frustum with optimum height can obtain an optical absorption above 95% over a wide wavelength of 200 to 380 nm and a broad angle of incident light between 0 deg and 70 deg. All these findings not only show that the gradient Al component Al_xGa_{1  −  x}N material has a great potential advantage for the UV photocathode, but it also provides an efficient broadband and omni-directional light trappers for the UV photocathode.