Abstract: Objective The purpose is to investigate effects of ultrasonic vibration assisted TIG on formation characteristics, microstructure, and mechanical properties of HG70 high strength low alloy steel weld. Methods Taking HG70 high strength low alloy steel as test material, ultrasonic vibration is assisted during tungsten inert gas (TIG) welding process to form a stimulated vibrating welding arc. By observing microstructure and measuring hardness of welded joints, influence of ultrasonic vibration assistance on metallographic structure and mechanical properties of welded joints is analyzed. Based on drilling hole method, evolution trend of residual stress in welded joints with and without ultrasonic vibration assistance is also tested. Results The experimental results show that peak hardness of TIG weld is 238.2 HV, which increases to 259.1 HV after ultrasonic vibration assistance, an increase of about 9%. Peak longitudinal residual stress of TIG weld is 425 MPa, which is reduced to 338 MPa after ultrasonic vibration assistance, a decrease of about 20%. Conclusion By assisting periodic ultrasonic vibration to welding arc to control crystallization process of welding pool, grains are refined, microstructure of welded joints is improved, and residual stress in the weld is reduced.