Abstract: To suppress the undercut defects during the dissimilar material welding of EH40 high-strength steel and 304 stainless steel, the application of magnetic field assisted K-TIG welding technology was investigated. A multimodal magnet-controlled power supply was developed, which can generate stable DC magnetic field and sinusoidal alternating magnetic field with a frequency ranging from 10 to 100 Hz. A series of welding experiments were carried out under zero magnetic field, DC longitudinal magnetic field, DC pulsed longitudinal magnetic field and sinusoidal AC longitudinal magnetic field. The influence mechanism of magnetic field modes on the weld formation of EH40/304 dissimilar steels was systematically analyzed. The results indicate that the Lorentz force generated by the DC and pulsed-DC magnetic fields effectively balances the magnetization force difference between the dissimilar materials, thereby correcting the deviation of molten pool metal toward to the EH40 side and significantly suppressing the undercut. Under the influence of an alternating longitudinal magnetic field with a strength of 5 mT and a frequency of 20 Hz, the undercut phenomenon is improved. Furthermore, the periodic electromagnetic stirring effect also refines the weld grain, significantly enhances the impact toughness of the welded joint.