Abstract: A transient three-dimensional model of welding arc in CO₂ gas shielded arc welding was founded.With the experimental data of transient welding current,the basic theory of magneto hydrodynamics (MHD) and the coupling of multi-physics function of ANSYS,the distribution of the current density in electric field,the electromagnetic force in magnetic field,the temperature and velocity in the flow field were simulated.The simulation result for arc temperature filed is basically identical with the experimental data in the reference.In addition,the laminar hypothesis and incompressible assumption in the model were verified by computing the Reynolds number and Mach number.The numerical model can provide theoretical guidance with the controlling of welding arc in the CO₂ gas shielded arc welding, and it also lays a foundation for the further study on the analysis of transient CO₂ arc.