Abstract: The control of oxygen content and composition and the transformation conditions of non-metallic inclusions in gear steel are calculated with the thermodynamic calculation software Factsage. The results show that the composition of non-metallic inclusions with high plasticity in 20CrMoH steel is as follows (mass fraction):SiO₂ 0% to 10%,Al₂O₃ 22% to 55%,CaO 42% to 60%,and MgO 5% to 10%. Under this condition,the mass fraction of aluminum in molten steel is above 0.020%,that of calcium is above 0.7×10^-6,and the activity of oxygen in molten steel is about 0.000 5%. If the compositions of refining slag with CaO > 40%, Al₂O₃ ≤ 37%, MgO 10%, (% CaO +% MgO)/% SiO₂=10,and the SiO₂ content as low as possible are selected,Al₂O₃ and MgO·Al₂O₃ in molten steel can transform into calcium aluminates with low melting point. It is found that the contents of TOat the end of LF and RH refining both decrease with the increase in slag basicity,and the content of TOdecreases with increasing the content of Al₂O₃ in the slag. In the refining process,the composition of the non-metallic inclusions changes in order of "inclusions of the Al₂O₃ system→inclusions of the MgO-Al₂O₃ system→inclusions of the CaO-MgO-Al₂O₃ system",among which the change of inclusions of the MgO-Al₂O₃ system to inclusions of the CaO-MgO-Al₂O₃ system is inwardly from the outside surface and the reaction speed is relatively slower. Thus,at the end of the ladle furnace refining (LF),there are still many unchanged MgO-Al₂O₃ system inclusions. The TOcontent of the steel significantly affects the composition change of the inclusions and decreasing TOcontent favors the change of inclusions.