Abstract: The mechanism of melting behavior of metal under ultrasonic-tungsten inert gas (U-TIG) welding was studied. The results indicate that U-TIG welding can significantly enhance the arc force and electromagnetic convection, change the metal flow behavior, and increase the penetration and the ratio of depth to width. This method extends the application range and improves production efficiency of ordinary TIG welding. Besides, the mechanism of arc force enhancing and penetration increasing were also presented in combination with experiment and theoretical analysis.