Abstract: Narrow-gap TIG welding of the 30 mm thick titanium alloy TC4 plates was carried out,and microstructures and mechanical properties of the joint were analyzed,including metallographic microstructure,tensile strength,impact toughness and micro hardness.Through optimizing the welding process test,the appearance and the protection effect are good,the narrow gap welded joints are obtained with free of macroscopic defects.The microstructure of the weld is mainly composed of lamellarαphase,βphase and fine needle martensiticα’.The arc of the narrow clearance in TIG welding of magnetic controlled arc is oscillating periodically in narrow gap slope,which can reduce the welding heat input while guaranteeing the full fusion of the side wall,the resulting thermal influence zone is narrower than in the conventional TIG welding method,and the width is about 1-2 mm.In the tensile test of the joints,the fracture position is in the weld seam,the fracture mode is ductile fracture,the joint tensile strength can reach up to 96% of that of the parent material.The hardness of heat affected zone is the highest,that of the weld is the lowest,the hardness peak of the whole joints appears in the coarse grain zone of the HAZ,but no obvious hardening and softening zone were found.