Abstract: Based on ultrasonic assisted TIG (U-TIG) and conventional TIG welding tests of TC4 titanium alloy, microstructure and texture of welded joints were analyzed by metallographic microscopy and electron backscatter diffraction, and microhardness of welded joint was tested. After that, the two kinds of specimens were tested under axial load with a stress ratio of −1 by a high-frequency fatigue testing machine. The results showed that primary β grains of conventional TIG welded joints were easy to form coarse curved columnar crystals in the form of association-crystallization across welding layer, and α′ phase had obvious growth orientation. However, in the U-TIG welding process, primary β columnar crystals were broken, trend of associated crystallization was curbed, acicular α′ martensite size was refined, average grain size in the weld was reduced, grain size gradient distribution was reduced and average microhardness was increased. With the aid of ultrasound, grain growth along the heat dissipation direction was inhibited, and α′ martensite orientation strength at <0001> was significantly decreased. In addition, fatigue limit under the stress ratio of −1 was increased from 360 MPa to 387 MPa, and fatigue strength of welded joints was increased by about 7.5%.