In order to solve the problems of low impact energy and high dispersion in the core of S355G10+N steel plate used for offshore wind power with 85mm thickness, the microstructure of the steel plate with different thickness was studied by metallographic examination, scanning electron microscope and energy spectrum analysis combined with theoretical calculation of inclusion formation.The results show that the microstructure of the steel plate is mainly granular bainite, a little ferrite and a little pearlite.The surface of the plate is homogeneous with the thickness of 1/4, but severe segregation and lath bainite are found in the heart of the plate.The impact fracture morphology shows that the MnS and (Nb,Ti)C inclusions generated by center segregation are fracture sources, and the fracture mode near them is intergranular fracture. The lath bainite in the center segregation of steel plate increases the crack propagation, and the combined action of inclusions and lath bainite significantly reduces the impact toughness of steel plate core.