The invention provides a forming die and method for a magnesium/aluminum alloy bimetal composite pipe, and belongs to the technical field of metal plastic working. The forming die for the magnesium/aluminum alloy bimetal composite pipe comprises a main shaft, an extrusion gasket, an extrusion core pin, an extrusion cylinder lining, an extrusion cylinder, an extrusion die, a die-filling extrusion shaft, a cylindrical roller bearing, a thrust bearing, a gear A, a gear B, a sleeve, a gearbox, an end cap A, an end cap B and a shearing device. The extrusion cylinder, the extrusion cylinder lining,the main shaft and the extrusion core pin simultaneously move to the right to realize the extrusion forming process, furthermore, repeated rotation of the die-filling extrusion shaft in the extrusionprocess is realized through transmission of the gear A and the gear B, and after the extrusion is completed, extrusion remnant materials are cut off by the shearing device. Advantages of high pressuretorsion and extrusion forming are combined, blank is simultaneously subjected to extrusion and torsion actions, so that the metal flow resistance, the preheating temperature of the blank and the diecan be reduced, and the interface quality and dimensional accuracy of the composite pipe is improved.