Abstract: Use fiber laser to weld TC4 titanium alloy under vacuum and atmospheric environment, observe the shape and structure of the weld seam obtained by welding with different laser power in atmospheric and vacuum environment, and test the performance. The results show that laser welding of the vacuum environment is more favorable. The vacuum environment can significantly improve the weld penetration, increase the depth-to-width ratio of the weld, suppress the spatter during welding, and greatly reduce the porosity in the weld. There is little difference in the weld structure between the atmospheric environment and the vacuum environment, and the width of the heat affected zone of the vacuum environment is significantly reduced. In both environments, the microhardness of the weld is saddle-shaped and the weld has the highest hardness. The average hardness of the weld in the atmospheric environment is about 390 HV, which is higher than the average hardness of the weld in the vacuum environment of 360 HV. The tensile strength of the weld was tested The tensile test piece was completely fractured at the base material. The tensile strength of the test piece was about 960 ~ 980 MPa.