<p>With the development of lightweight and precision, joining of small-diameter metal tubes, such as aluminum (AA) and stainless steel (SS), is increasingly required in high-end equipment manufacturing industries such as automobiles. Limited space and significant property differences make it difficult to obtain reliable bonding between small-diameter AA and SS tubes. Under such severe geometric constraints, conventional impact welding techniques often face challenges related to insufficient loading efficiency and process instability. This work investigates underwater electrical explosion welding (UEEW) as a feasible solid-state joining approach for the small-diameter AA and SS dissimilar metal tubes. An UEEW experimental platform was set up to achieve controllable underwater electrical explosion. The 9-mm-diameter SS 304 tube and 7-mm-diameter AA6063 tube welding experiments were carried out at discharge energies of 1.75, 2.52, 3.43, and 4.48 kJ. Well-bonded tubular joints were achieved when the discharge energy was 2.52 kJ, corresponding to a discharge energy condition derived from the impact loading requirement associated with the welding window. SEM and EDS were used to analyze the bonding interface. The results show the presence of wavy and vortex structures at the bonding interface with localized elemental diffusion, and intermetallic compounds have been found in some areas. These results demonstrate that underwater electrical explosion can provide effective impact loading for joining small-diameter dissimilar metal tubes and verify the feasibility of UEEW for tubular joining under limited geometric scales.</p>