Based on the density functional theory of the first-principles calculation, properties of non-metallic inclusions in steel were calculated, including structural properties such as the formation energy and binding energy, electronic properties such as the energy band, the density and the work function, mechanical properties such as the elastic modulus, the Poisson’s ratio, and the Vickers hardness. The influence of inclusions on mechanical properties and corrosion properties of steel were discussed. Non-metallic inclusions possessed a spontaneous formation trend. Titanium nitride possessed the minimum formation energy of − 3.899 eV, which were the most easily formed. Inclusions of MnS possessed the largest formation energy and was relatively difficult to form. The minimum binding energy of TiN was − 7.051 eV, and the structure of TiN after formation was the most stable. The band gap of Al₂O₃ was 5.832 eV, which belonged to non-conductive insulation material. The band gap of MgO, CaO and CaS were 4.482 eV, 3.651 eV and 2.368 eV, respectively, belonging to semiconductor materials. The bulk modulus, shear modulus and Young's modulus of TiN were the highest. The Vickers hardness of MgO and TiN were 37.49 GPa and 24.57 GPa, respectively, which were relatively large, and the Vickers hardness of CaS inclusions was the smallest. Inclusions of TiN and MgO had a great influence on the strength and hardness processing properties of steel. According to the value of B/G and Poisson's ratio, inclusions were unfavorable to the toughness of steel and less elastic than steel matrix. The average work function of inclusions followed the sequence of TiN > Fe > MgO > CaO > CaS > MnS. Inclusions of TiN and MnS possessed the greatest influence on pitting corrosion.