Abstract: Pure copper samples with different grain sizes were prepared by regulating the annealing process, the influence of grain size on the impact work and fracture mechanism of pure copper was studied. The relevant impact absorption data were obtained through the Charpy impact test, the microstructure and morphology were characterized for the samples before and after the shock. The results show that with the increase of the grain size from 9 μm to 3009 μm, the impact energy decrease from 5.61 J to 3.38 J, which indicates a negative relationship between the impact energy and grain size. The pure copper microstructure with the same volume, the smaller the grain size, the higher the grain boundary density. During the impact deformation, more additional dislocations are accumulated at the crystal boundary, and a large part of the impact kinetic energy is converted into the deformation storage energy. At the same time, these crystal boundaries with a large number of dislocation are easy to generate microholes in the process of further impact deformation, which induces smaller pores through the micropore aggregation mechanism.