Abstract: TC4ELI, a typical damage-tolerant titanium alloy, is widely used in pressure-resistant structures of deep-sea submersibles. Taking 8 mm thick TC4ELI plate as the object, the microstructure evolution, mechanical properties, crack initiation and propagation behavior of the TIG welded joints were investigated under low, medium and high heat input conditions. The results show that with the increase of the heat input, the length and content of the acicular α' phase in weld increase, and the thickness of the lamellar α phase increases. The acicular α' martensite in the weld enhances grain boundary strengthening and dislocation pinning, raising the strength and hardness of the joint, and the ultimate tensile strength of the joint increase from 890.5 MPa at low heat input to 936.2 MPa at high heat input. The work-hardening rate of the joint at high heat input exhibits a stable increase over a wider true strain range, which is attributed to the fact that the thick lamellar α phase in the weld prolongs the dislocation slip path, thereby enabling more dislocations to participate in the plastic deformation process. As the strain increases, the strain concentration zone gradually shifts toward the bottom of the weld. The weld width exhibits a characteristic of being wider at the top and narrower at the bottom along the plate thickness, and the bottom demonstrates weaker deformation capacity compared to the middle and top sections during tension, leading to a higher susceptibility to strain concentration at the bottom. The fracture surface shows a ductile fracture mode with dimples and strip-like tearing ridges. Further analysis indicates that thin lamellar α phases allow cracks to penetrate directly, whereas thick lamellar α phases promote crack propagation along α phase interfaces, increasing crack resistance and contributing to improved elongation. This work provides valuable insight into the microstructural evolution and fracture failure behavior of TC4ELI titanium alloy TIG welded joints.