Laser lap welding was conducted on steel/aluminum dissimilar metals in the experiment. The effects of rare earth addition layer on the microstructure, element distribution, precipitated phases and mechanical properties of interface transition zone were carefully studied. The results showed that the welded joints with good macro/micro shape can be obtained under laser power of 2.0 kW, welding speed of 3 m/min and defocusing amount of +2 mm. The study also found that the addition of rare earth Y in the layer can effectively regulate the metallurgical reaction of steel/aluminum interface and promote the formation of various ductile intermetallic compounds such as Fe23Y6 and Al3Y. To a certain extent, the precipitation of ductile phases at interface can inhibit the generation of brittle phases and effectively improve the mechanical properties of the joints.