In order to investigate the superfast deformation mechanism of metal, the high-current pulsed electron beam (HCPEB) technique is employed to irradiate the polycrystalline pure copper. The microstructure of the irradiated sublayer is investigated by using transmission electron microscopy. It is suggested that the stress with very high value and strain rate is introduced within the sublayer after HCPEB irradiation. The dislocation cell and the tangle dislocation formed by cross slip are the dominant defects after one-pulse HCPEB irradiation, whereas, dense dislocation walls and twins are the central microstructures after five- and ten-pulse irradiation. The diffusion and the climb of the atomic plane can cause the formation of the steps at the grain boundary and (or) the twin boundary. Based on the structure characteristics of the irradiated surface, the possible deformation mechanism induced by HCPEB irradiation is discussed.