Defects have been found on the class 1 BOSS welds in Loop 1 pressure boundary in domestic PWRs,such as,incomplete fusion,slag inclusion,blowhole and so on. an analysis is made from the aspects of the type,position and reason of the defects,two treatment solutions are formulated,one is grinding and repairing,one is cutting and replacing. Through the design and development of special tools,the technical difficulties,including the control for grinding residual wall thickness,the control for cutting to prevent foreign matters and the argon protection for weld back,have been conquered. All of these are successfully applied in Ningde,Fuqing,Fang Chenggang and Changjiang NPPs,which provide reference for the treatments of the welding defects of BOSS and other similar structures.