Abstract: The shape parameters of the double ellipsoid heat source were mostly determined by measuring the shape of the molten pool, which existed a problem of high cost and low efficiency. Aiming at the problem, the shape parameters of the double ellipsoid heat source under different welding parameters, heat source parameters and plate thickness conditions were calculated by simulating the butt joint model of thin plates below 6 mm in this paper. Based on the simulation results, the mathematical model functions between the shape parameters of the double ellipsoid heat source and the plate thickness were established, which simplified the shape parameters of heat source, improved the efficiency of welding simulation, and then reduced the experimental cost.