Abstract: In response to the limitations of current laser shock welding conducted under atmospheric conditions, which can adversely affect weld quality, a laser shock welding system operating in a vacuum environment was developed. Comparative experiments were performed on Cu/SS304 dissimilar metals under both vacuum and atmospheric conditions. The surface morphology, microstructural characteristics of the weld interface, elemental distribution, and various mechanical properties of the welded specimens were systematically investigated. The results show that welding in a vacuum environment significantly enhances the success rate of laser shock welding experiments compared to welding in an atmospheric environment. Moreover, the rebound defect at the weld center in atmospheric environments is eliminated, resulting in a substantial increase in the effective welding area. Consequently, the mechanical performance of the welded joints improves notably. In addition, a more pronounced waveform is observed at the Cu/SS304 interface under vacuum conditions, which further contributes to superior welded joint performance. Specimens welded in a vacuum environment exhibit higher maximum breaking tension than those welded in an atmospheric environment. The laser high-speed shock welding approach under atmospheric conditions proposed in this study provides a pathway for optimizing the microstructure and mechanical properties of dissimilar metal joints, thereby enhancing the overall welding quality.