Abstract: To produce 9Ni steel plates with excellent microstructure, properties andplate shape in wide and thin gauge via a medium-heavy plate mill, the pack rolling process was investigated through experiments and numerical simulations. The chemical composition of 9Ni steel was designed, and the CCT curve of the tested steel was determined. Through pilot trials, 3 mm thick pack-rolled plates were obtained alongside suitable welding methods for stacking and spacer compound formulations. Finite element simulations were used to analyze stress and strain distribution patterns under different reductions to establish reasonable rolling schedules. Results indicate that uniform stress distribution at 10 mm reduction, while reductions greater than 15 mm caused non-uniform distribution with significant differences between the lower surface of the top plate and its upper surface.There is higher stress at edges but relative uniformity centrally. Industrial production on the medium-heavy plate mill implemented roughing rolling start at 1 150 ℃, finishing rolling start at 950 ℃, and finishing rolling end at 810 ℃. According to the CCT curve, cooling rates greater than 5 ℃/s yield fully martensitic microstructure. The Ac₁ and Ac₃ of the tested steel are 621 ℃ and 735 ℃, quenching temperature must exceed 735 ℃ for complete austenitization, while two-phase region quenching above 621 ℃ achieves reversed austenite. The 5 mm thick wide and thin gauge plates exhibited excellent comprehensive mechanical properties with flatness less than 3 mm/m, meeting standards and customer requirements.