The 20-mm-thick 316LN austenitic stainless steels are welded by the 20 kW ultra high power fiber laser, the effects of welding parameters on the macro-morphologies and weld formation are investigated, and the microstructures and mechanical properties of welded joints are analyzed. The results show that better surface appearances can be obtained via a negative defocusing. The weld microstructure is single austenite and there exist the equiaxed grains in the central areas of the top and bottom zones of welds; in contrast, there exist the coarse columnar grains in the central areas of the middle zone of welds. Under the optimal welding parameters, the tensile strength of welded joints is 645 MPa, equivalent to that of base metals. The fracture of welded joints occurs at the fusion line in a typical ductile manner. The micro-hardness of the heat-affected zone is slightly higher than those of welds and base metal.