Abstract: Negative pressure cavity valve for coolant flow rate in the primary loop main pipeline of nuclear power plants connects with the main pipeline by the way of welding. In this paper, the treatment of weld defect before a negative pressure cavity valve for 30 MW unit in Qinshan Nnuclear Power Plant has been taken as an example, comprehensive description has been made from the preliminary analysis of the causes of weld crack defects, the propose and demonstration of online casing sealing welding repair scheme, as well as technical difficulties in its implementation. Based on further analysis of the morphology and microstructure of the weld, the primary cause of the weld crack is confirmed. On this basis, technological change have been taken to solve the effect of the insufficient structure on equipment operation, and to avoid the occurrence of weld crack defects, thus ensure the safe and stable operation of the unit.