In order to improve the photoemission characteristics of the GaAs photocathode, GaAs nanostructures (nanoholes and nanowires) with three sublayers and Al component of gradient distribution are designed in this article. Finite element method was used to study the effects of Al component gradient interval, component range, height, diameter, and other parameters on light absorption of nanostructures. And the quantum efficiency of these nanostructures was calculated. The results show that the wide band absorption can be achieved with small gradient interval and component variation range, and the nanostructure can enhance the light capture ability with certain angle inclination. In the gradient component nanostructures, the thickness of the top most layer has a great influence on the quantum efficiency. Highlights The nanohole and nanowire structures with three sublayers and Al component gradient distribution were designed. The effects of gradient interval, range of Al components, and geometric parameters on the optical properties of nanostructures are discussed.