The invention discloses a processing method for an automatic production line of rock wool decorative boards and a composite forming milling cutter, and belongs to the technical field of advanced processing and manufacturing of forming cutters, which can increase the convenience in processing the rock wool decorative boards. The composite forming milling cutter is assembled in the sequence of a chamfer milling cutter, a first adjusting pad, a side milling cutter, a second adjusting pad and a middle milling cutter, adjustment is performed by virtue of the adjusting pads, and processing is performed by adopting the milling cutter. The production practice shows that the problems of long processing cycle, low production efficiency, high manual labor intensity and the like of the rock wool decorative boards are effectively solved by adopting the method and the milling cutter.