Abstract: The electron beam welding test is conducted on the pressure-resistant shell of a deep submersible from the 60 mm thick TC4 ELI. The study aims to investigate the weld formation, microstructure, and mechanical properties of the welded joints by different process parameters. The relationship between the microstructure and mechanical behavior of the weld at different depths under typical parameters is analyzed to elucidate the variation law of the microstructures and mechanical properties of the electron beam welded joints of the thick titanium alloy. The results show that a joint with smooth weld surface, full penetration on the backside and good mechanical properties could be achieved with the acceleration voltage of 60 kV, welding beam current of 490 mA, welding speed of 500 mm/min, and focusing current of 2 060 A. The microstructures from the center of the weld to the base metal are sequentially columnar α' phase, equiaxed α' phase, (α'+α+β) mixed phase, and (α+β) duplex phase. The average tensile strength of the joint reaches 900 MPa, which is equivalent to that of the base metal, the percentage elongation after fracture is 8%, 75% of that of the base metal,and the average impact toughness value reaches 53.8 J/cm², higher than that of the base metal. There are regular changes in the microstructures of the upper and lower parts of the weld, which has little effect on the mechanical properties.