In this paper, COMSOL Multiphysics Commercial Package software is used to simulate the models of two kinds of GaN nanostructure arrays, and to study how the geometric structure and periodicity of arrays affect the optical properties of GaN nanostructures. By analyzing the schematic diagram of electric field distribution and absorption curves of GaN nanostructure arrays, we concluded that the absorptivity is a decreasing function of periodicity in non-homogenous shaped nanostructures. In addition, a gentle change in geometric structure or having a lower effective refraction index on the incident side of the light are more conducive to enhancing the light absorption ability of GaN nanostructure arrays. Simulation experiments on GaN nanostructures will provide some references for the structural design of photocathodes, which will help UV detectors to achieve efficient light absorption.Nomenclature