Abstract: In the mass production of ultra-thick plates of offshore high strength steel manufactured from steel ingots, a common problem is that the performance at the 1/4 thickness position is good, while the impact toughness at the 1/2 core position is poor, resulting in significant economic losses. To address this problem, the mechanical property tests and finished product chemical composition analyses were performed at different thickness positions of the steel plates. Microstructure and inclusion characteristics at 1/4 thickness and 1/2 core positions were comparatively investigated by means of an optical microscope and scanning electron microscope. The results show that the chemical composition deviations at different positions of the steel plates with a deficiency in impact toughness at the 1/2 core position are not significant, and the content of non-metallic inclusions is limited. The primary reason for the reduced impact toughness in the core is the coarse prior austenite austenite grains and the high fraction of granular bainite after tempering. In the subsequent rolling process, the rolling reduction is increased to enable effective transmission of rolling force to the 1/2 core position. Meanwhile, the cooling rate of the steel plates during quenching process is raised to address the problems of the coarse prior austenite grains and the high proportion of granular bainite after tempering. As a result, the impact toughness at the core position of ultra-thick offshore high-strength steel plates achieves a remarkable enhancement.