The present disclosure relates to the technical field of dissimilar welding of Cu and a steel, and in particular to a welding wire for dissimilar welding of Cu and a steel and a preparation method thereof and a method for welding Cu and a steel. The present disclosure provides a welding wire for dissimilar welding of Cu and a steel, including, in percentages by mass, 5-25% of iron phase, less than 0.1% of inevitable impurities, and copper matrix. The welding wire of the present disclosure, containing two elements, i.e. copper and iron, is conducive to the mixing of the two phases—copper and iron—during the welding process, to form a mutual soluble region, thereby makes it possible to greatly increase the weldability, reduce the width of the weld, effectively overcome the tendency of cracks, and thus to ensure the formed weld with a high crack resistance.