Side top plate friction stir welding tool and welding method

The side top plate friction stir welding tool comprises a bottom plate, fixing devices and a friction welding base plate, the friction welding base plate is arranged on the center line of the bottom plate, at least two sets of fixing devices are arranged on the portions, on the two sides of the friction welding base plate, of the bottom plate with the center line as the symmetry axis, and the fixing devices are provided with jacking pressing heads moving in a reciprocating mode. A cushion block on the jacking pressure head is matched with a backer at the side part of the friction welding base plate, so that the side top plate is tightly pressed and fixed on the friction welding base plate, the backer is detachably arranged on the friction welding base plate, and the cushion block is detachably arranged on the jacking pressure head; on the basis of an existing welding tool, the connection relation between the backer and the friction welding base plate and the connection relation between the base plate and the