The lock butt joint of 0Cr15Ni5Cu4Nb stainless steel of 7 mm in thickness was welded with high power fiber laser, the characterizations of welding process were studied by analyzing the influence of different welding parameters on the weld quality, mechanical properties and microstructure. Result shows that porosity is the main defect in the weld, and the negative defocus and high welding speed can prevent porosity from formation effectively. There is an increase in the hardness of the weld, and the top and bottom weld have higher hardness than the central weld does. The tensile strength of the joint is 970 MPa, and higher than that of base metal, but its impact toughness is slightly lower, is 89% of that the base metal. Weld microstructure consists of martensite, retained austenite and δ
-Fe, and the central weld has more δ
-Fe than the top weld. Martensitic transformation was not sufficient in the central weld, the microstructure of HAZ is mainly quenched martensite with small grain.