Abstract: Taking magnesium alloy weld pool as the research object, a three-dimensional mathematical model of the weld pool under moving heat sources was established. The electromagnetic field analysis is coupled with the heat flow analysis using the finite element software ANSYS. The temperature distribution and the velocity vector distribution of the fluid flow in magnesium alloy weld pool were simulated under the effect of the external magnetic field and without external magnetic field. The simulation results show that the electromagnetic force induced by external magnetic field driving the molten liquid metal occurs a rotary motion, which changed the original movement of liquid metal and heat transfer. The fluid flow velocity and flow range was therefore increased, with increased weld width and reduced depth. The experimental results verify the simulation results.