Dissimilar metal butt joints of 1 mm thickness 6061 aluminum alloy and 1 mm 304 stainless steel are welded with filler material by fiber laser welding-brazing method. The mathematical model of relationships between welding parameters and tensile strength is established based on the response surface methodology. Effects of different parameters on the tensile strength are discussed. Afterwards, the tensile strength is used as an optimization goal and the optimum welding parameters are determined with the aid of the numerical optimization method. In order to test the suitability of the mathematical model, the optimized process parameters are used to carry out the welding experiment. The interaction effects of welding parameters on the tensile strength is significant. The results indicate that the maximum tensile strength of the welded joints can reach 180 MPa and the thickness of the intermetallic compounds is almost 2.85 μm.