Dissimilar metal butt joints of 2.5 mm thick 6013 aluminum alloy and galvanized low carbon steel are welded by fiber laser wire fusion brazing method. The microstructure of the fusion brazing welded joints is investigated by optical microscope, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The joint strength is measured by tensile test. The effect of heat input on interface intermetallic compounds of aluminum/steel butt joint and joint strength are discussed. Experimental results show that butt joints with good appearance and certain tensile strength can be obtained under appropriate welding parameters. Further analysis reveal that this kinds of intermetallic compounds on the interface are mainly FeAl2 and FeAl3. With the increase of heat input, the thickness of the intermetallic compound is also increased. The uniform distribution of Al-Si eutectic along grain boundaries on α-Al substrate shows in welded seams. The maximum tensile strength of welded joints with a 30° groove is 88 MPa, the strength can reach up to 135 MPa when using a 45° groove.