Abstract: The friction plug welding repair technology for damaged threaded hole of QT400 ductile iron motor base of rail transit and the joint performance were studied. The dimensions of plug hole and plug rod were designed, and the welded joint was prepared by friction plug welding. The microstructure, hardness of the repaired joint was analyzed. New threaded hole was machined on the joint, and the static load tensile property of the threaded hole was tested. The results show that an effective metallurgical connection is formed between the repair area and the cast iron matrix, and the interface at top, side wall and bottom of the plug hole are well bonded without defects.The number and size of spherical graphite in the weld center area are greatly reduced compared with those of the base metal, and the diffusion of carbon element promotes the formation of pearlite, which leads to the increase of hardness and strength of the weld center.The static load tensile load of thread hole of the plug welded joint is 71%higher than that of thread hole of the base metal.