The notch in the middle of the original friction stir welding （FSW） joint was patched to simulate the weld defects. The weld defects in the middle position of the FSW joint were repaired by Tungsten Inert Gas （TIG）. The influence of the repair welding process on the microstructure and mechanical properties of the joint was studied to provide technical support and follow-up engineering guidance for the performance evaluation of the FSW weld of the tank. The results show that the TIG weld zone of the repaired joint is mainly composed of coarse equiaxed dendrites as compared to FSW joint.At the interface between the FSW joint and repaired joint coarse equiaxed grains and refined equiaxed grains are intermingled. The solid- solution zone of the repaired joint is greatly affected by the thermal cycle with coarsened grain， while the overaging zone is less affected by the thermal cycle with the occurance of over-aging phenomenon and the presence of a softening zone. The tensile strength and elongation of the repaired joint are decreased as compared to the original FSW joint. The hardness distribution is substantially "W" shape with the lowest hardness in the WNZ. Crack initiation of the repaired joint occurs at the weld toe displaying a large number of dimples on the fracture surface. Meanwhile， second phase particles are detected in the dimples showing a typical ductile fracture mechanism.