Abstract: Instability during laser welding of titanium alloys is not an incidental phenomenon, which results from the fluctuation of welding process parameters. It is a sort of inherent disciplinein in laser welding possesses. The penetration depth and width of weld undulate irregularly and intensively under some conditions and the welding mode transforms between conductive and penetrative ones. So there should be two critical energy points for laser welding of titanium alloys. When the welding process is in the range of conditions in which the Penetrative hole forms, laser-induced Plasma absorbs laser energy in a fluctuant state. At the same time, the vaporization procedure of titanium alloys presents unbalanced speciality. These two factors cause instability in the laser welding of titanium alloys. Based on the mechanism of instability and characteristics of energy transmission during laser welding of titanium alloys that are deeply analyzed in this thesis,the calculative pattern of critical conditions for laser welding stability of titanium alloys is finally deduced.