Abstract: Low vacuum laser welding technology was used to weld 40 mm thick Ti6Al4V alloy, and the microstructure and mechanical properties of different positions were compared and analyzed. The results show that the base metal is composed of equiaxed primary α phase and β transition microstructure. The microstructure of the heat affected zone is α phase, residual β phase and quenched quasi-steady α' martensite. The microstructure of the weld fusion zone mainly includes α' martensite with different sizes and distribution states and α phase formed at slow cooling rate. The average tensile strength of welded joints is 988 MPa, and all the fracture positions are in the base metal.The average impact absorption energy is 28.8 J in the upper and middle weld zone, significantly better than 24.8 J in the lower weld zone.There are elongated and highly dense α' martensite at the bottom of the weld fusion zone to degrade the impact toughness of the material. In contrast, the short and coarse α' martensite structures formed in the middle and upper part of the melting zone have higher impact toughness.This paper provides data support and relevant theoretical basis for the connection of Ti6Al4V alloy plate and the further improvement of mechanical properties of joint.