The invention provides a friction-stir welding method for generating excess weld metal through an additional top plate. The method includes the steps that first, a welded workpiece is clamped, the top plate is placed on the top, and a stirring head is aligned with the butt-connection face of the welded workpiece after the assembly; second, the stirring head is rotationally pressed downwards and penetrates through the top plate to be pressed into the welded workpiece till a shaft shoulder of the stirring head makes contact with the top plate, and then the stirring head is moved to perform friction-stir welding; third, the workpiece is detached after welding and clamped on a milling machine, and a milling cutter is aligned with a welding joint of the top plate after the assembly; fourth, the milling amount is adjusted, the milling is performed only one time, the separated top plate can be taken down easily after the milling, and welding assemblies generating excess weld metal are acquired. According to the method, the thinning effect of a friction-stir welding joint is eliminated by adding materials, the excess weld metal of the welding joint is generated, the bearing capacity of a connector is improved, and the method can be applicable to connection between aluminum alloy, copper alloy and magnetism alloy, and special materials.