<p>Due to the vacuum environment and the high aspect ratio, electron beam welding has significant advantages in the welding of thick titanium alloy equipment and is widely used. As a result, addressing root defects in electron beam welding of thick titanium alloy plates has become a hot topic both domestically and abroad. This paper focuses on studying the location and morphology of root defects during deep penetration welding of titanium alloy. The causes of these defects are analyzed, and the underlying mechanism is revealed. Additionally, measures are proposed to eliminate and control the occurrence of root defects, ultimately improving the quality of electron beam welding in titanium alloys. The results indicate that in the welding of thick titanium alloys, deep penetration welding is prone to have spiking and porosity defects. When the surface tension and hydrostatic pressure near the root of the weld pool keyhole are lower than the metal vapor pressure, it becomes difficult to maintain keyhole wall stability. As a result, the liquid metal collapses into the keyhole, and the metal vapor becomes trapped under rapid cooling conditions, leading to the formation of spiking defects. Additionally, if the liquid metal takes longer to fill the keyhole from the raised equilibrium position to the bottom than the upper keyhole closing time, the weld root is likely to develop chain porosity defects. Taking measures to balance vapor pressure with surface tension and hydrostatic pressure, or achieving complete penetration to the back, can effectively reduce and eliminate spiking and chain porosity defects, ultimately resulting in a high-quality weld. A backside overflow groove pad structure has been designed not only eliminate spiking and chain porosity defects, but also achieve a high-quality weld.</p>