The welded joint of TC4 titanium alloy is easy to form coarse grain, and there is a large residual stress and residual deformation after welding. By applying ultrasonic aid in the conventional TIG welding process, the ultrasonic amplitude converter is directly contacted with the TC4 titanium alloy plate, and the ultrasonic cavitation effect generated by ultrasonic vibration and the mechanical vibration stirring effect are used to influence the flow and solidification process of liquid metal in the weld pool, so as to refine the microstructure and grain of the weld and reduce the residual stress of the weld. The grain size and residual tensile stress of welded TC4 titanium alloy joints under different ultrasonic power were measured. Compared with conventional TIG welding, the average grain size of the weld is reduced by 22.70 % and the average residual stress release rate is 82 % when 2000 W ultrasonic power is applied. Grain refinement can increase grain boundary number and improve material strength. Reducing the residual stress of weld can further reduce the fatigue crack propagation rate of weld. Therefore, it is of practical significance to improve the weld microstructure and reduce the residual stress of welded joints by ultrasonic assisted welding technology to improve the strength and reliability of welded structures.