3.0 mm thick BTi6431S high temperature titanium alloy was welded by fiber laser beam welding. The effect of welding parameters on welding quality and stability was studied from the aspects of weld formation, porosity, and characteristics of metal vapor/plasma. The results indicate that the laser power has an effect on the weld width, residual height, underfill, and porosity. The welding speed mainly affects the weld width and weld width ratio. The effect of welding process on weld quality is closely related to the characteristics, and stability of metal vapor/plasma. At lower laser power (1.5-2.5 kW) or higher welding speed (3.5-4.5 m/min), the metal vapor/plasma area is large and area fluctuation is obvious, the weld is not penetrated, and the porosity is high. Under moderate laser power (3.0-3.5 kW), and welding speed matching (2.5 m/min), the area and periodic fluctuation of metal vapor/plasma are small, the weld is fully penetrated, and the forming quality is good.