Abstract: In order to study the welding quality of partial penetrated laser overlap-welding of thin steel sheet, SUS304 austenitic stainless steel and SPCC cold-rolled steel sheet were welded with fiber laser and CO2 laser. And the main factors affecting the partial penetrated laser welding were analyzed. The micro-structure and the mechanical properties of the weld were measured, finally, the effects of main welding parameters, including laser power, welding speed, defocus distance, pulse frequency and duty ratio, on weld surface quality and performance were obtained. The results show that under the proper welding condition, the weld topography is uniform and continuous, the strength of weld is 50% higher than that of base metal, and its micro-hardness of fusion zone is higher than that of base metal.