Abstract: In order to meet the rating requirement for inclusions in the steel,the additive amount of calcium to the steel was controlled through thermodynamic calculation,and after calcium treatment,Al₂O₃ was modified to low-melting-point 12CaO·7 Al₂O₃ which could be removed from 20CrMo steel.Thermo-Calc software was used to simulate the precipitation process of calcium aluminates including CA6,CA,and CA2 and MnS inclusions during the solidification of liquid steel.SEM was employed to analyze the spindle-shaped inclusions.It was found that a small amount of inclusions had dark-colored core consisting of Al₂O₃ and CaS etc,and light-colored outer layer consisting of MnS inclusion.It indicates that in the solidification process of liquid steel,composite inclusions precipitated earlier such as Al₂O₃ and CaS can be captured by MnS precipitated later as its core.