The composition change of slag and alloy phases were investigated during the process of selective reduction by thermodynamic calculating the selective reduction of the slag components with different modified slag composition, addition of reducing agent and reduction temperature. The results showed that the process of the selective reduction will be promoted by raising the temperature and the ratio of modifier to slag. The solubility of SiO2 increased as temperature increased. Selecting the appropriate amount of modifier will promote the selective reduction. In the first stage, Fe was reduced from the slag, followed by the reduction of P, Mn and Si. Finally, Fe-Si-Mn metal was obtained as a result of the selective reduction.