Abstract: Ti/steel clad plates are considered ideal candidate materials for applications such as marine engineering and low-temperature pressure vessels operating at −70 °C. However, their large-scale production and application are severely restricted by the manufacturing process and the quality of interfacial bonding. To investigate the influence mechanism of vacuum pressure on interfacial morphology and mechanical properties, TC4/09MnNiDR Ti/steel clad plates were fabricated using explosive welding under ambient pressures of 20 kPa, 60 kPa, and 100 kPa. A specially designed vacuum chamber was employed to precisely control the environmental pressure during the explosive process. After welding, all specimens were subjected to a uniform stress-relief annealing treatment at 550 °C for 2 h under a vacuum atmosphere to eliminate residual stresses and improve the reliability of subsequent characterization. The interfacial microstructure and chemical characteristics were systematically analyzed using multiple characterization techniques. Scanning electron microscopy (SEM) was used to observe interfacial morphology, including wave formation and defect distribution. Energy dispersive spectroscopy (EDS) was applied to examine elemental distribution across the bonding interface. Electron backscatter diffraction (EBSD) was employed to evaluate grain structure, grain boundary characteristics, recrystallization fraction, and texture evolution near the interface. Electron probe microanalysis (EPMA) was conducted to quantitatively determine the chemical composition of the interfacial melting zone and vortex regions, with both mapping and point analyses performed to identify the constituent phases. Mechanical properties at −70 °C were evaluated through tensile tests, Charpy impact tests, and three-point bending tests, all carried out in accordance with relevant national standards, with the results reported as mean values ± standard deviations. Results show that the vacuum environment significantly improves interfacial quality. With increasing vacuum degree, the interfacial wave becomes finer, more continuous, and more uniform, while the thickness of the molten layer decreases, and the presence of defects and brittle intermetallic compounds is reduced. Electron backscatter diffraction (EBSD) analysis reveals grain refinement and an increased recrystallization fraction at the interface under vacuum conditions, demonstrating that the interfacial microstructure can be effectively controlled by adjusting the ambient pressure. Electron probe microanalysis (EPMA) further reveals that the vortex regions are mainly composed of Fe and Ti elements, while the weld seam region exhibits a stable chemical composition with Ti:Fe atomic ratios close to 1:1 or 2:1, indicating that the dominant intermetallic phases formed at the interface are TiFe and TiFe₂. Benefiting from the optimized interfacial structure, the clad plates exhibit excellent mechanical properties at −70 °C. The tensile strengths under 20 kPa, 60 kPa, and 100 kPa are 880 MPa, 911 MPa, and 867 MPa, respectively. The impact absorbed energies are 17.5 J, 10.2 J, and 6.4 J, and the flexural strengths are 1469 MPa, 1350 MPa, and 1167 MPa, respectively. This study demonstrates that vacuum explosive welding is a reliable technique for producing high-performance low-temperature metal composites, with 60 kPa identified as the optimal processing window.