Abstract: A fiber laser was used to weld the 3.5 mm-thick TC4 titanium alloy. The microstructure and mechanical properties of welded joints were investigated, and the laser welding parameters were optimized under the experimental condition.Resultsshowed that the fusion zone was mainly composed of acicular α′ martensite and a small amount of β phase, and that the heat-affected zone (HAZ) was composed of α phase and a small amount of α′ martensite. Microhardness changed slightly in fusion zone (FZ) , but decreased obviously in HAZ. When the laser power was 4.0 kW and the welding speed 3.0 m/min, the welded joint with excellent mechanical performance could be obtained, and the tensile strength of joint was close to that of base metal. There were many dimples distributed in the fracture surface of the joint, and it presented the obvious characteristics of ductile fracture.