Abstract: During the transient multiphase flow simulation of high-current GTAW, to accurately describe the interaction between the arc and molten pool, the arc heat and arc pressure should be loaded on the dynamic interface between the metal phase and gas phase. Therefore, a real-time marking method of phase interface is proposed. The maximum value of phase volume fraction gradient is searched by traversing the grid data in the 3D computing domain to identify and mark the single-layer mesh zone near the phase interface. The volume heat source and volume force are transformed by dividing the surface heat source and surface force by the grid thickness. They are loaded at the marked zone as the arc heat and arc pressure. The result shows that this method has strong robustness to accurately mark the GTAW phase interface with fluctuation. The marking time is only 7.34% of the governing equation iteration time, which meets the real-time requirements. The digging effect of arc pressure on the molten pool and the penetration effect of arc heat along the depth direction is accurately simulated. Without using the combined heat source, the finger penetration characteristics of high-current GTAW are obtained in the simulation results.