In order to understand the phenomenon that solid inclusions are easier to be removed in contrast to liquid inclusions during steelmaking process, a mathematical model was established to describe the separation processes of octahedral and plate-like inclusions based on the analysis of the forces acting on inclusions at the steel-slag interface. Besides drag force, buoyancy force and capillary force as well as added-mass force considered in the previous mathematical models, the interfacial resistance force caused by the deformation of the steel-slag interface was further considered in the model of this study. Based on the model, the effects of interfacial tension between phases (steel, slag and inclusion) and slag viscosity on the separation process were studied. The results showed that the capillary force was the driving force for solid inclusion crossing the interface between steel and slag, and it was determined by the overall wettability of the system (steel, slag and inclusion) and the shape of an inclusion. Meanwhile, the drag force was the resistance force for the separation of an inclusion at the steel-slag interface, and the slag viscosity was an important factor to this force. Without the consideration of chemical dissolution, the released interfacial free energy of the steel-slag-solid inclusions system was a driving energy for the separation of solid inclusions at the interface, and it would be sufficient to make most solid inclusions separate from the interface. Furthermore, the Gibbs free energy released during the dissolution of inclusions was much larger than the interfacial free energy. According to the principle of minimum free energy, the dissolution process would promote the entering of solid inclusions into slag. Therefore, the Gibbs free energy was also a driving energy for the separation process. Considering the separation time was very short (less than 10-3 s), in steelmaking process, solid inclusions would be removed quickly at steel-slag interface, once they contact with the steel-slag interface.