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Due to low hardenability, the conventional SA508Gr.3 steel cannot meet the design requirements of increasing wall thickness of forgings for nuclear power pressure equipment. In this paper, the hot processing and manufacturing technology of SA508Gr.4N steel ultra-thick forgings for the new generation of nuclear reactor pressure vessel was introduced. The engineering development of nuclear SA508Gr.4N steel forgings with wall thickness from about 700mm to 1000mm was realized by China First Heavy Industry (CFHI). The largest forging was made of 228 tons ingot and the final maximum dimensions reached about ϕ4000mm × 1000mm. The samples were taken anatomically from the forging at various thickness and radial positions, and subjected to the tensile, Charpy impact and Drop-weight tests. As a result, the full-section property of the forging met the requirements of the ASME code. Compared with nuclear SA508Gr.3 steel forgings, the strength and toughness of SA508Gr.4N forgings were greatly improved. Meanwhile, based on the fact that the actual property value is much higher than the requirements of ASME code, higher design property requirements of SA508Gr.4N steel forging was proposed. In addition, the nuclear neutron-irradiation-embrittlement data of SA508Gr.4N steel, which were cut from the actual engineering forgings and experimented in a high flux engineering test reactor (HFETR), was obtained. The main research and development goals for engineering application of SA508Gr.4N steel in the future were also discussed.