Abstract: Three types of weld defects of slag inclusion,the lacking of penetration and local thinning were defined by the MMM characteristics which were obtained by the magnetic memory testing of pressure vessels on the locations of prefabricated defects,and were verified by X ray examination,showing the effectiveness of MMM qualitative analysis on three types of defects.After hydrostatic test of pressure vessels,residual stress is released that is to cause magnetic memory testing signal to be weakened.Through analysis of these signals one can however find the macro expansion cracks appearing in some weld defects,which can be verified by polishing the welds.