During the welding process of the lower wall cylinder of CRDM anti-seismic bracing structure of HPR1000 integrated surfacing top structure,there are two repairs welded continuously. During the last repairs welded to remove defects,more welding cracks were found,which led to the whole scrap of the lower cylinder. The causes of weld defects in lower cylinder are analyzed in detail,and then measures such as changing the design of weld groove,optimizing welding parameters,changing the position of weld groove and adopting integral forging are taken. The improvement measures proposed in this paper have important reference significance for the following welding of HPR1000 and AP1000 nuclear power unit integrated reactor top structure.