Abstract: A defect was found during the flaw detection of 18MND5 steel plate.Low magnification inspection, microstructure inspection, in-situ analysis, thickness direction performance and scanning electron microscope inspection were carried out on the defect parts.The results show that there is serious segregation in the central area of the thickness of the steel plate, and it contains lots of non-metallic inclusions, which is the main reason for flaw detection defects.The purity of the steel is improved by adjusting the content of P and S in the steel and protecting pouring.By reducing the superheat of molten steel and controlling the constant-speed pouring, the segregation grade of the cast slab is reduced.At the same time, a reasonable rolling process is formulated to improve the inspection pass rate of the steel plate.