A double-shaft-shoulder friction stir welding stirring head with four sets of auxiliary devices comprises an auxiliary pull rope, a traction device, an auxiliary current heating device, a fixed pulley, an auxiliary multi-degree-of-freedom robot with a welding seam tracking device and a flexible coupler. With the four sets of auxiliary devices, the stress condition of the stirring head can be greatly improved, the service life of the stirring head is prolonged, and the welding length of the stirring head is increased. A slope is additionally arranged on an introduction plate, so that the reduction of the pre-tightening force between the two shaft shoulders after the pre-tightening force encounters softened metal in the working process is not reduced, and tunnel type welding defects can be reduced. Notches are formed in the corners and the positions with different thicknesses, so that the double-shaft-shoulder friction stir welding stirring head is particularly suitable for friction stir welding of plane and space curve butt welds, linear butt welds and segmented part transverse butt welds of medium-thickness steel parts and steel parts which are slightly staggered in height and are equal in thickness such as various ships, various warships including aircraft carriers, conventional submarines, nuclear submarines, deep diving submersibles, large cylinders, large platforms, large flat plates and large transporters. The invention also provides a corresponding welding method.