Abstract: 00Cr12Ni10MoTi steel is extensively utilized in critical applications such as aerospace, energy and power systems, and high-end bearing manufacturing owing to its superior strength, hardness, corrosion resistance, and high-temperature performance. Nitrogen and oxygen contents exert a significant influence on the quantity, size, morphology, and distribution of non-metallic inclusions, which in turn dominate the mechanical properties of the material. In this study, thermodynamic analysis was performed on the performed of typical inclusions in 00Cr12Ni10MoTi steel. The results show that the actual solubility product of Al₂O₃ is larger than its equilibrium solubility product at 1 873 K enabling preferential precipitation in molten steel with a higher precipitation tendency in high nitrogenoxygen steel. In contrast, the actual solubility products of TiN and AlN are both smaller than their equilibrium solubility products, so they cannot precipitate at this temperature. Combined with Thermo-Calc simulation results, TiN mainly precipitates at the final stage of solidification and its mass fraction peaks at approximately 1 150 K, which is consistent with the thermodynamic analysis. Microstructural observation and analysis of inclusions in 00Cr12Ni10MoTi steel were carried out using scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) and electron backscatter diffraction (EBSD). It is found that the density, average size and area fraction of inclusions in high nitrogenoxygen steel are higher than those in low nitrogenoxygen steel, and the average grain size of high nitrogenoxygen steel is 7.9 μm which is 9.5% finer than that of low nitrogen-oxygen steel. Mechanical property tests indicate that the yield strength, tensile strength and impact energy of low nitrogenoxygen steel are 655 MPa, 918 MPa and 163 J, respectively, while those of the high nitrogen-oxygen steel reach 718 MPa, 961 MPa, and 165.5 J, respectively. Owing to the synergistic strengthening effect of dispersion strengthening by nitrogen-oxides and grain refinement strengthening by nitrogen, the mechanical properties of high nitrogen-oxygen steel are significantly better than those of low nitrogenoxygen steel. This study reveals the influence law of nitrogen and oxygen contents on inclusions and mechanical properties of 00Cr12Ni10MoTi steel, providing a theoretical basis for the stable production of highstrength and hightoughness martensitic stainless steel.