Abstract: An appropriate Cr content can enable Ni-Co based ultra-high strength hull steel to achieve a good strength-toughness combination, but its strengthening and toughening mechanisms have not yet been fully elucidated. This study investigated the effects of Cr content on the microstructure,secondary phase precipitation behavior,and mechanical properties of Ni-Co based ultra-high strength hull steel using techniques including scanning electron microscope（SEM）,X-ray diffraction（XRD）,physical-chemical phase analysis,and transmission electron microscope（TEM）,combined with room-temperature tensile and low-temperature impact tests. Microstructural analysis indicated that the microstructure of the test steel consists of tempered martensite and a small amount of reversed austenite. As the Cr content increased,the martensite start temperature（M_s） of the steel decreased,austenite stability improved, and its volume fraction increased. Phase analysis and TEM results reveals that the main precipitates in the Ni-Co steel are M₂C phases,and the Cr content has a significant impact on the size of the secondary phases. At 2%Cr,the low lattice mismatch between the M₂C phase and the matrix promotes the precipitation of a large number of M₂C phases smaller than 5 nm. At 3%Cr,numerous M₂C phases larger than 200 nm are observed,indicating significant coarsening. Mechanical test results demonstrate that the yield strengths of the 1Cr,2Cr,and 3Cr steels are 1 317,1 258, and 1 137 MPa,respectively. The addition of Cr reduces the solid solubility of C in the matrix,promoting the precipitation of nano-sized M₂C phases. With increasing Cr content,the decrease in the solid solution strengthening increment exceeds the increase in the precipitation strengthening increment,resulting in a gradual reduction in yield strength. The impact energies at -84 ℃ for the 1Cr,2Cr,and 3Cr steels are 6,42,and 49 J,respectively. Cr improves the low-temperature toughness of the Ni-Co steel primarily due to the increased reversed austenite content,which enhances crack blunting,and also because Cr promotes the precipitation of coherent nano-sized M₂C phases with the matrix, which reduces strain accumulation and stress concentration, thereby effectively suppressing crack initiation. The 2Cr steel,with its abundant nano-sized M₂C phases and a certain amount of austenite,exhibits a good combination of strength and toughness.