Abstract: T-joints of Ti-6Al-4V alloy were fabricated by double-sided synchronized laser beam welding with homologous filler wire. The influence and characteristics of the weld defects such as undercut, lack of fusion, and porosity defects during tensile process were investigated by the method of infrared thermal imaging. Results show that the temperature increased at the base metal on both sides of the skin. The stress concentration of T-joint was obvious with the increasing of deformation. The temperature and plastic deformation increased at the zone of stress concentration where the fracture occurred finally. The lack of fusion and porosity defects were located at center position of the hardening zone, the influence of strengthening effect for welded T-joint had little change. The temperature increased in base metal on one side of the skin because of plastic deformation, and the fracture occurred in base metal at the peak temperature.