Abstract: Gallium nitride (GaN) films are widely used in radiation environments, including aerospace, aviation and the nuclear industry etc., due to their excellent photoelectric properties and structural stabilities. However, the irradiation effects of energetic ion in GaN films are unclear and need to be clarified. In this work, microstructures, lattice strains, transmittance, and optical band-gap energy of GaN films induced by 8 MeV ¹²⁹Xe¹⁹⁺ ion at different fluences were investigated by high-resolution X-ray diffraction (HRXRD) and ultraviolet-visible (UV-Vis) spectrum analysis techniques. The HRXRD results indicate that ¹²⁹Xe¹⁹⁺ ion irradiation with low fluences induces a dynamic annealing effect in GaN films, leading to a shrinkage in the diffraction peak's full width at half maximum (FWHM). As the irradiation fluence increases, the diffraction peaks split and gradually shift toward lower angles, accompanied by a broadening in its FWHM. These suggested that high-fluence Xe ion irradiation causes lattice distortion, expansion, and the formation of damaged crystal layers in GaN films. Moreover, by analyzing the lattice strain and the thickness of the diffraction layer in GaN films, we observed that the thickness of the diffraction layer initially increases slightly and then decreases with the irradiation fluences. And the gradually increasing lattice strains lead to lattice expansions, twisting and distortions of the GaN films. The results from UV-Vis spectra show that the transmittance initially increases slightly and then gradually decreases with the ion fluences. Simultaneously, the optical band-gap energy first increases from 3.40 eV to 3.42 eV and then decreases to 3.19 eV as the ion fluence increases. In addition, after high-fluence irradiation, a pronounced absorption peak appears near 450 nm (~2.75 eV).