Abstract: The bypass-current plasma-MIG hybrid welding is a new process by coaxial coupling of the shunt arc achieved between the conductive copper nozzle of the plasma torch and the welding wire and the main arc between the wire and the base material. This process not only maintains the high efficiency of MIG welding but also provides more schemes for precise control of the welding process energy through the regulation of the bypass current. In order to understand the physical mechanism of action of the hybrid arc and the molten pool under this process, a hydrodynamic transient model was developed and verified with a reasonable experiment to compare the heat and mass transfer behavior inside the gas and liquid phases, and at the interface between them before and after the bypass-current loading. The results show that the maximum temperature of the arc decreases by about 1 000 K compared to that without the bypass current, and the effective heat flux at the interface of the hybrid arc and the molten pool decreases overall; the liquid metal flow rate inside the molten pool decreases significantly, and thus leading to a decrease in both dimensions of the penetration and the width; the maximum electromagnetic force direction of the hybrid arc and the molten pool remains unchanged, but the values are reduced.