The surface border of weldpool in MIG welding is irregular and the temperature in the pool is very high and is distributed extremely nonuniformly.The violent motion occur under the action of the electromagnetic force,the buoyancy force and the surface tension,etc.According to the bimodal distribution mode for arc current density with a large deformation of the pool surface,the calculation mode for electromagnetic force is developed and described in this paper.Numerical analysis is used to study the hydrodynamics behavior in the weldpool and the effects of welding technical parameters on fluid flow.The experiments show that the predicted results are in good agreement with experimental measurements.