31mm thick TC4 Ti-alloy plates were welded by magnetically controlled narrow gap TIG, the surface residual stress distributions were measured by indentation strain-gage method before and after vacuum post-welding heat treatment (PWHT). The results show that the levels of longitudinal and transverse residual stress is all high on the weldment surface. The residual stresses on the lower surface are little higher than that on the upper surface, especially for transverse residual stress. The residual stresses peak value of the weldment exists in the high temperature heat affect zone, which reaches 50%-60% of the material yield strength. After 650°C vacuum post-welding heat treatment, longitudinal and transverse stresses on the weldment surface are significantly reduced, maximum stress reduction is more than 50%, the residual stress peak value left on the surface doesn’t exceed 200MPa, and the original welding residual stresses on the surface go to redistribute.