To meet the high-performance requirements of copper/stainless steel composites for the "double-box lap joint structure" in nuclear fusion devices, this study employed explosive welding to fabricate thick TU0/316L composite plates. The interface characteristics and their correlation with performance were investigated through microstructural characterization and mechanical testing. Results demonstrate that jet interference and wave impedance differences under explosive impact induced a periodic wavy interface (wavelength: 789 μm). Molten mixing dominated in wave crest regions while solid-state diffusion prevailed in trough regions, with deeper diffusion observed in troughs. Dynamic recrystallization occurred in TU0-side grains, whereas 316L-side grains refined to submicron scale. The interface achieved remarkable shear strength (210 MPa) and pull-off strength (230 MPa), both exceeding the copper matrix"s ultimate strength. Bend testing showed no interfacial delamination, confirming superior peeling resistance. This research provides critical insights for interface design and process optimization of composite components under extreme nuclear fusion conditions.