Abstract: Aiming at the problems of low arc pressure and shallow weld penetration in twin-electrode TIG (T-TIG) welding, a T-TIG welding method assisted by cusp magnetic fields with different polarities is introduced. A high-speed camera and an infrared camera were used to study the influence of cusp magnetic fields of different polarities on arc shape and weld characteristics, and a physical model was constructed to reveal the interaction mechanism between cusp magnetic fields and arc plasma. The results show that the external cusp magnetic field affects the shape of the T-TIG arc and the temperature field of the weld, and the cusp magnetic fields of both polarities can refine the structure of the heat-affected zone. Among them, under the action of a positive cusp magnetic field, the T-TIG arc shape changes more greatly, the weld temperature field is more concentrated, the penetration depth of the weld is increased by 37.1% compared with that without a magnetic field, and the energy utilization efficiency increases by 31.6%.