<p>TC4ELI titanium alloy is widely used in marine engineering, shipbuilding, and other fields due to its high strength, good toughness, and excellent corrosion resistance. This study investigates the welding process of 13.5-mm-thick TC4ELI titanium alloy using automatic TIG welding with tungsten electrode oscillation. The arc heat in non-oscillating automatic TIG welding is more concentrated, which can lead to significant welding deformation, high residual stress, and coarse microstructure during the welding process. Therefore, arc oscillation welding is employed to achieve a more uniform heat distribution within the narrow gap, thereby reducing welding deformation and enhancing the strength of the welded joint, thus improving overall performance. The results show that high-speed camera analysis reveals that oscillating arc promotes more uniform fusion within the narrow gap and a more uniform overall temperature distribution. The droplet transfer mechanism in non-oscillating-arc welding is characterized by large-droplet transfer, whereas the oscillating arc, influenced by the circumferential force, pushes the molten droplets to blend with the side walls. Consequently, the welding residual stress in oscillating-arc welding is reduced within a uniform temperature gradient. The oscillating arc can refine the acicular <i>α</i>′ martensite structure and eliminate some microstructural defects. The tensile strength of the welded joint increases from 995.14 to 1022.99&#xa0;MPa, and the impact toughness rises from 59 to 73&#xa0;J/cm² compared to non-oscillating-arc welding. The microhardness is also enhanced, achieving a synergistic improvement in mechanical properties.</p>