In special applications in nuclear reactors and deep space environments, gallium nitride detectors are subject to irradiation by α-particles. Therefore, this work aims to explore the mechanism of the property change of GaN material, which is closely related to the application of semiconductor materials in detectors. This study applied molecular dynamics methods to the displacement damage of GaN under α-particle irradiation. A single α-particle-induced cascade collision at two incident energies (0.1 and 0.5 MeV) and multiple α-particle injections (by five and ten incident α-particles with injection doses of 2 × 1012and 4 × 1012ions/cm2, respectively) at room temperature (300 K) were simulated by LAMMPS code. The results show that the recombination efficiency of the material is about 32% under 0.1 MeV, and most of the defect clusters are located within 125 Å, while the recombination efficiency of 0.5 MeV is about 26%, and most of the defect clusters are outside 125 Å. However, under multiple α-particle injections, the material structure changes, the amorphous regions become larger and more numerous, the proportion of amorphous area is about 27.3% to 31.9%, while the material’s self-repair ability is mostly exhausted.