Abstract: electron beam welding characteristics of TC4 titanium alloy were investigated,and the unfilled,porosity and other defects and its causes are also analyzed. The results show that the defect of unfilled was related to the focus mode of the electron beam,and the focus mode of the electron beam have an improving effects on the unfilled; The spherical stomatal in the weld joints is caused by the offset between the electron beam and the contact surface; the main cause of the formation of bellmouth and"ear-shaped"holes is plasma.