Abstract: Objective The purpose of the paper was to study microstructure, microhardness, and mechanical properties of welded joints in order to realize high efficiency and high-quality welding of titanium alloy. Methods Taking 3 mm thick TC4 titanium alloy as test material, butt welding experiment of was carried out by YAG laser welding method. Results The results showed that laser welding of TC4 titanium alloy could obtain well-formed welded joints without defects of cracks and pores. Microstructure of weld was typical net basket microstructure, consisting of a large number of αʹ martensite phase and a small amount of high-temperature residue β phase, heat affected zone was acicular αʹ martensite with staggered arrangement. Microhardness of welded joints showed a hump distribution, and gradually increased from base metal to heat affected zone. Microhardness of internal zone of weld was about 385 HV, which was greater than that of base metal (about 360 HV). This was due to martensite transformation of weld during solidification under the action of thermal cycling, which was caused by transformation of β to acicular αʹ phase. Fracture position of welded joints was in the base metal. Fracture morphology showed morphology of equiaxial dimple with different sizes, which was a ductile fracture mode. Bend and impact tests indicated that welded joints had good plasticity and toughness. Conclusion In summary, laser welding of TC4 titanium alloy could achieve good formation of weld, which could be promoted and applied in related engineering.