Abstract: Slow strain rate tensile method was adopted to evaluate the stress corrosion crack (SCC) sensitivity of 100 mm ultra-thick TC4 titanium alloy electron beam welded (EBW) joints under artificial seawater. The corrosion mechanism was studied by observing the microstructure and fracture morphology of the joint. The results showed that the base metal have no SCC sensitivity in seawater. However, the upper, middle and lower parts of the weld showed slight SCC sensitivity at room temperature under the condition of strain rate ε=1×10^-6 s^-1. In seawater, anodic dissolution accursed in the weld metal, which increased the Ti ion concentration in the solution at the crack tip, then, H atoms are evolved and absorbed at the crack tip. The diffused hydrogen then promotes dislocation emission and increases the dislocation density at the α' phase boundaries and in the α' phase. The crack initiation and propagation are results of the accumulation of highly hydrogen. At the same time, hydrogen enhances mobility of dislocations, then the crack propagates at a lower stress level.