An in-situ solid state repairing method for the failure flat clinched joint was proposed to reconstruct the appearance and improve the mechanical properties. The microstructure, mechanical properties, and fracture failure behavior of the flat clinched and repaired joints were comparatively investigated to clarify feasibility and effectiveness of this method. Results indicate that the fractured neck of failure flat clinched joint is successfully repaired in-situ. The microstructure of the weld nugget is sufficiently refined, which enhances the mechanical properties of the repaired joint. Rotation speed and dwell time play crucial roles on the mechanical properties of joints. Compared with the initial flat clinched joint, the mechanical properties of the repaired joint are strengthened by the metallurgical bonding. The repaired joint has 409.90% higher maximum shear tension strength and 68.92% longer maximum extension at fracture. In addition, the fracture failure behaviors demonstrate that the repaired joint is much stronger than the flat clinched joint. This method for repairing the failure flat clinched joint is proven to be feasible and effective.