Abstract: In order to study the effect of activating agent and high frequency magnetic field on arc behavior and the change mechanism of penetration depth, A-TIG welding of Q235 Steel was carried out with fly ash as activating agent, the high frequency longitudinal magnetic field is applied in the welding process to realize the combined effect of the two. It is found that the penetration depth increases most obviously when the coating amount is 0.3 mg/mm², and the longitudinal magnetic field of 1.5 kHz is introduced under the coating amount, the penetration depth reaches 2.70 mm, which is 1.80 times of that of the conventional argon arc welding, but the penetration depth will decrease with the increase of magnetic field frequency. By collecting the data of arc shape, current density, arc force and arc temperature during welding, the mechanism of combined action of activating flux and high frequency longitudinal magnetic field on TIG welding is discussed, it is found that the combined action of the active agent and the magnetic field can change the distribution of the external magnetic field in the plasma, causing the charged particles to be more Lorentz force, causing the arc to contract further within a certain range, compared with magnetron-TIG, it can improve the “Magnetic suction” phenomenon caused by high frequency magnetic field and make the arc heat more concentrated.