Abstract: An experimental system of bypass-current MIG-TIG double-sided arc welding was established. A highspeed camera was used to monitor the arc shape, and "orifice-igniting" method was developed to measure the distribution of arc pressure with and without bypass current. Results showed that after the bypass current being added, the coupling arc expands in volume to be bell-shaped and decreases in gradient of arc diameter along the arc length under the action of Lorentz force. Compared with the single MIG arc, the pressure of coupling arc decreases overall and distributes more uniformly. In the arc center, the coupling arc pressure was only 29.4% of the single arc, which leads to uniform weld appearance and improves the welding quality.