Abstract: In order to improve the castability of low carbon Al-killed steel, based on the thermodynamic calculation of low carbon Al-killed steel refining slag, the effect of typical components CaO/Al₂O₃ (C/A), SiO₂ and FeO on the liquidus temperature, viscosity and surface tension at 1 650 ℃ in the refining slag was studied. Combining the test results of the physical and chemical properties (melting temperature and viscosity–temperature curve) of the refining slag, a dynamic control model for the argon-blowing top-up refining slag was established and implemented in production practice. The results show that as C/A increases, the melting point of the refining slag increases when SiO₂ mass fraction is 5%, while it first decreases and then increases when SiO₂ mass fraction is 10%. Meanwhile, with the increase of C/A, the viscosity of refined slag and the inflection temperature of the viscosity −temperature curve also rise. When the composition range of refined slag is C/A 0.75−1.50, SiO₂ mass fraction of 0−10%, and FeO mass fraction of 2%−6%, lower liquidus temperature, higher viscosity, and higher surface tension can be obtained. The model established in production practice can achieve the dynamic addition of slag-forming materials, significantly improve the stability of physical and chemical properties of refining slag, and significantly reduce the standard deviations of C/A and total iron (TFe) mass fraction in refined slag. The average values of liquidus temperature, viscosity and surface tension at 1 650 ℃ decrease by 6.9 ℃, increase by 0.020 6 Pa•s, and 0.0109 N/m, respectively. The average mass fraction of total oxygen (TO) decreases from 2.78×10⁻³% to 2.25×10⁻³%, an improvement of 19.1%, and the castability of molten steel is significantly improved.