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 T[O]at the end of LF and RH refining both decrease with the increase in slag basicity,and the content of T[O]decreases 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 T[O]content of the steel significantly affects the composition change of the inclusions and decreasing T[O]content favors the change of inclusions.