The invention provides a repair welding method and a welding tool of a stirring friction plug for friction preheating of a telescopic shaft shoulder, which relates to a repair welding method and a welding tool of a filling type stirring friction plug. The invention solves the problems that in the repair welding process of the filling type stirring friction plug, strong resistance causes a stirring needle to be broken when the stirring needle does not completely plastic deformation so that the continuous fluidity of materials is poor. The method comprises the following steps: (1) taking the same materials for the stirring needle and the repair welded part; (2) punching a conical defect repair preset hole; (3) determining the size of the stirring needle; (4) positioning the conical defect repair preset hole; (5) preheating by friction; and (6) carrying out repair welding. The welding tool is characterized in that an upper circular ring on the telescopic shaft shoulder is connected with a lower cylinder on a clamping body of the welding tool through a key, a spring is arranged between the upper end surface of a lower circular ring on the telescopic shaft shoulder and the lower end surface of the lower cylinder, the lower cylinder is connected with the outer surface of the lower circular ring by a limiting plate, and a stirring body is arranged in a clamping hole on the clamping body of the welding tool and a through hole on the telescopic shaft shoulder. The invention is used for stirring friction welding of welded seam keyholes and repair welding of defects.