The invention provides a welding method for performing aluminum/steel welding by using three-beam laser. The welding method comprises the following steps of 1) selecting a welding base metal and protective gas; 2) grinding the surfaces of an aluminum alloy plate and a steel plate by using abrasive paper; 3) performing alkali washing and acid washing on the polished aluminum alloy plate, wiping thealuminum alloy plate with alcohol, and drying the aluminum alloy plate; wiping the polished steel plate with acetone, and drying the steel plate; 4) fixing the aluminum alloy plate and the steel plate on a clamp; 5) welding by adopting three laser beams, namely a laser beam I, a laser beam II and a laser beam III, and 6) inspecting and analyzing after welding. The welding method has the advantages that the welding process is simple, the deformation amount of base metal is small, the weld joint forming is improved, the welding time is shortened, the production efficiency is improved, and the production cost is effectively saved.