Abstract: In this paper, a method of applying transverse magnetic field to the arc is proposed to change the arc heat distribution on both sides of the weld by deflecting the arc, so as to improve the weld forming quality. In order to understand the influence mechanism of applied transverse magnetic field on arc heat input distribution, the rule of GTAW arc heat input distribution was studied. A three-dimensional model of GTAW arc pool coupling in dissimilar steel fillet weld under applied transverse magnetic field was established, and the coupling calculation of arc temperature field, electromagnetic field and flow field was carried out. Two kinds of arc heat distribution methods of deflecting the torch and external transverse magnetic field deflected arc were compared and analyzed. On the basis of the above work, the effect of magnetic induction intensity on the penetration form and the heat distribution of arc under the applied transverse magnetic field deflected arc was studied. The results show that the adaptability of deflection welding gun is poor in fillet welding, while the deflection arc with applied transverse magnetic field can obtain better welding quality. The relevant research can provide technical support for the optimization of welding process parameters of this kind of dissimilar steel fillet welding.