Abstract: In response to the limitations of current laser impact welding conducted under atmospheric conditions, which can adversely affect weld quality-this study establishes a dedicated laser impact welding system operating in a vacuum environment. 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 compared to welding in air. Moreover, the rebound defect commonly observed at the weld center in atmospheric conditions was eliminated, resulting in a substantial increase in the effective welding area. Consequently, the mechanical performance of the welded joints improved notably. In addition, a more pronounced waveform was observed at the Cu/SS304 interface under vacuum conditions, which further contributed to superior joint performance. Specimens welded in a vacuum exhibited higher maximum breaking tension than those welded in air. The high-speed laser impact welding approach proposed in this study provides a promising pathway for optimizing the microstructure and mechanical properties of dissimilar metal joints, thereby enhancing the overall welding quality.