The microstructural evolution of TIG welded joint in Ti-6.5Al-2Zr-1Mo-1V alloy treated by thermo hydrogen processing was investigated by optical microscopy, transmission electron microscopy and scanning electronic microscopy and the mechanical properties of welded joints were tested and analysed. The results showed that δ-hydride formed after hydrogenising at 700°C for 3 hours, and it is a face-centered cubic crystal. And these hydrides brought a strong fuzzy effect on microstructure. A lot of rhombic martensite phase α" generated in the area of welded joint. Metastable β(H) phase and α" phase were gradually decomposed to α and δ phases during aging. Hydrides were dissolved, the hydrogen was removed from the alloy and the recrystallization occurred in the process of annealing in vacuum. After hydrogen treatment, the refinement effect of grains in the fusion area was very obviously, that is, coarse columnar grains changed to finer equiaxed grains. Non-equilibrium phase reduced in grains. After hydrogen treatment (hydrogenization at 700°C for 3 hours, eutectoid at 300°C for 8 hours and annealing at 750°C for 8 hours in vacuum), the tensile strength of welded samples increased by 25.7%, and the elongation increased by 37.6% compared with welded samples without hydrogen treatment. After hydrogen treatment, the surface morphology of tensile fracture was relative flatness.