Abstract: In order to study the combined effect of magnetic field and active welding influence on the welding pool of liquid metal flow pattern, the paper established the moving heat source under the action of three-dimensional transient mathematical model of magnesium alloy TIG welding. By using fluent software and its secondary development function to simulate the welding pool temperature field and velocity field under the effect of single magnetic field, single active welding and the combined effect of both. The simulation results show that liquid metal in the welding pool was directed clockwise rotation under the effect of single magnetic field, velocity field presented asymmetrical bimodal distribution，maximum speed biased the pool side. Under the effect of single active welding liquid metal in the welding pool formed outside-in convection mode, which scour the bottom of the pool to increase penetration. The fluid flow is more complex under the combined effect of magnetic field and active agent. The liquid metal flow near the pool surface is mainly affected by surface tension. The liquid metal flow inside the pool is mainly affected by external electromagnetic force.