Based on the actual weld shape from stainless steel sheets with non-penetration CO2 laser lap welding and fiber laser lap welding, by taking the Gaussian-plane+cylinder-body composite heat source model, the obtained heat source proportionality coefficients are 0.85 and 0.70, respectively. By correctly setting the other four parameters for describing heat source, including the radius and the depth of Gaussian heat source, and the radius and the depth of cylinder heat source, the change laws of weld shape characterization parameters (surface seam width, middle weld width and weld penetration) of lap welds can be obtained under different welding powers and welding speeds. The simulation results are well consistent with the experimental results.