To address the issues of low impact energy and high variability in the core of 85 mm thick S355G10+N steel plates used in offshore wind power, the microstructure of steel plates with different thickness was investigated through metallographic examination, scanning electron microscope, and energy spectrum analysis, combined with theoretical calculations of inclusion formation. The results show that the microstructure of the steel is predominantly granular bainite, with minor amounts of ferrite and pearlite. While the surface and the 1/4 thickness region of the plate exhibit homogeneity, significant segregation and lath bainite are observed in the core. Impact fracture morphology shows that MnS and (Nb,Ti)C inclusions generated by center segregation act as fracture initiation points, with intergranular fracture occurring around these inclusions. The presence of lath bainite in the center segregation zone increases crack propagation, and the combined effect of inclusions and lath bainite significantly reduces the impact toughness of the steel plate core.