Abstract: A three-dimensional (3D) numerical analysis model of tungsten inert gas welding arc with extra high-frequency longitudinal magnetic field was developed based on the local thermodynamic equilibrium assumption. With this method, the temperature field,velocity field and pressure field of arc plasma were investigated by solving the Maxwell equations, continuity equation, momentum conservation equation and the energy conservation equation. Combined with Faraday's law of electromagnetic induction, electromagnetic induction effect of extra high-frequency longitudinal magnetic field was taken into account. The mechanism of arc contraction in extra high-frequency longitudinal magnetic field was clarified successfully. Simulation results showed that extra high-frequency longitudinal magnetic field inducted circular electric field, it produced circular current, which interacted with the magnetic field to generate radial lorenz force, the arc contracted after all.