Aiming at the upper Ω seal weld of control rod drive mechanism（CRDM）in a nuclear power plant in China, primary investigation and overlaying repair technology have been carried out. Welding experiment is based on the representative test piece of 1:1 material,structure and restriction,and adopts automatic TIG welding machine. The overlaying parameters,key know-how and process flow have been achieved;by thin slice-radiographic examination,chemical analysis,metallographic examination and XRD phase analysis,the performance comparison between test piece and international typical witness piece has been done,the technical project and inspection item for overlaying piece performance examination has been established,the overlaying parameters and performance data of the upper Ω seal weld of CRDM has been obtained,so that a complete set of technology with performances equivalent to international typical witness piece’s performances has been achieved.