The invention aims to provide a welding wire for connection of aluminum / steel dissimilar alloy and a processing process thereof. The welding wire has a good comprehensive performance, adopts a special element to substitute the Si element, ensures good flow of solder and reduces the brittleness of the compound layer on the joint interface. In the technical scheme, the welding wire is prepared through adding such elements as Ag, Mn, Mg, Ti, Zr and Zn with Al-Cu as a matrix. The processing process of the welding wire comprises stock preparation, melting, component analysis, extrusion, wire drawing and cleaning. The invention has the advantages that: due to the high content of Cu element, the invention can inhibit the growth of the compound layer on the interface, improve the performance of the Al-Fe compound layer, and particularly improves the crack resistance of the compound layer; since the Ag element, the aluminum and the steel all have high affinity, the invention can enhance the flowing property of liquid solder in the steel surface; the Mn and Ti elements can improve the performance of the compound and the flowing property of the solder; the Mg and Zn elements can promote the flowing property of the solder and improve the strength of the welding wire; and the refined grain of Zr can also enhance the strength of the welding wire.