Dissimilar metal joining of steel/aluminum alloy by slab CO2 laser beam welding with filling wire is investigated and the influence of different heat input on the weld formation quality is introduced. The influence of different heat input on microstructure, phase and mechanical properties of joints are analyzed. The experimental results show that microstructure in the joints with higher heat input parameters compared with lower heat input parameters is no longer homogenous, Fe-Al intermetallic phase with the shape of strip and lump can be found in the joints, the thickness of the intermetallics layer increases in the interface of aluminum/steel joints, and the main phase in the melts of joints transforms from FeAl and solid solution of Al in α-Fe to Fe3Al and solid solution of Al in α-Fe. Mechanical properties deteriorate with the increase of the intermetallics layer. Tensile test shows that tensile (or shear) strength with high heat input parameters is 72.9% of that with low heat input parameters.