Abstract: Based on of the activity data of each component of a CaO-SiO₂-Al₂O₃-MnO system calculated by thermodynamic software FactSage, the formation condition of plastic inclusions in the CaO-SiO₂-Al₂O₃-MnO system was discussed by thermodynamic calculation. Comparison with some experimental data proved that this proposed method was feasible. It is shown that in order to acquire deformable inclusions in tire cord steel, the dissolved aluminum should be controlled less than 0.000 3%, and the dissolved oxygen in liquid steel must be controlled in the range of 0.002%~0.006%.