In order to find out the reason of unqualified impact at the center of thick quenched and tempered high strength ship plate, the composition and microstructure of the core sample were analyzed by means of spectrometer, microscope and SEMEDS. The results showed that the reason for the unqualified impact at the center was the presence of largesize MnS inclusions, Nb and Ti inclusions, as well as more Al2O3 inclusions and Mg, Ca and other foreign inclusions. The analysis suggested that, through the measures such as improving the cleanliness of molten steel, avoiding secondary oxidation pollution, controlling ladle furnace (LF) refining cycle and operation, reducing the degree of superheat of molten steel, optimizing the electromagnetic stirring and soft reduction process, and adopting the slow cooling for stacking, the internal quality of the slab could be improved, and meanwhile, the central segregation and inclusions could be reduced. At the same time, the core impact toughness could be effectively improved by properly optimizing the slab heating, rolling and heat treatment process.