The invention provides a method and a device for equivalent solid-phase repairing welding of a tank friction stir welding defect. A welded tank drum section is mounted between a machine head box and a machine tail box; the tank drum section passes through an embracing ring and a welding defect of the tank drum section is located on the embracing ring; a repairing welding host machine is fixed right above the embracing ring; a milling cutter is mounted on the repairing welding host machine; a through hole is arranged in a welding position with the defect on the tank drum section; an inner supporting rod set is arranged on a welding defect position in the tank drum section; a positioning pin is mounted on the repairing welding host machine, and then a welding base plate is in close contact with an inner wall of the tank drum section; the positioning pin is detached; a friction stopper rod is mounted on the repairing welding host machine; the friction stopper rod and the through hole in the tank drum section generate sufficient friction heat and plastic deformation. According to the method and the device, the repairing welding process is stable and reliable; the whole property of a repairing welding joint is equivalent to the property of a friction stir welding joint; a repairing welding device is used for effectively solving a repairing welding problem after the welding defect of a total assembling circular seam under a tank-forming state is detected.