Abstract: Mold flux plays a significant role in the continuous casting of steel. Especially, the viscosity (or its inverse, fluidity) of mold flux is a key parameter for industrial applications to aid in product quality. In this paper, viscosities of different types of fluorine-containing continuous casting mold fluxes were first measured by the rotating cylinder method, and then a new viscosity estimation model was established based on the Arrhenius equation combined with nonlinear regression analysis to analyze the influence of component changes on the viscosity. Combining model calculation and experimental measurement, an iso-viscosity diagram of the CaF₂–Na₂O–Al₂O₃–CaO–SiO₂–MgO slag system was also created. It is found that deviation within 10% is calculated using the model in this study compared with the traditional viscosity estimation models of different types of fluorine-containing continuous casting mold fluxes but gradually increases when the w(CaF₂) of slag exceeds 20%, mainly due to the change of slag composition caused by fluoride volatilization. Finally, the measured value cannot correspond to the composition of the initial slag, and the model cannot give an accurate estimated value. It is also found that an increase of CaF₂ can significantly reduce viscosity, whereas, the effect of Al₂O₃ and Na₂O on viscosity is restricted by CaF₂ content. When w(CaF₂) > 17%, the viscosity of slag decreases with increasing w(Al₂O₃), and when w(CaF₂) < 17%, the viscosity of slag increases significantly with increasing w(Al₂O₃). When w(CaF₂) > 11.5%, the viscosity of the slag system decreases significantly with increasing w(Na₂O) mass. When w(CaF₂) < 11.5%, the effect of Na₂O on viscosity is not obvious. In addition, the diagram shows that the mass fraction of CaF₂ in the low viscosity area is nearly 14%. This shows that the viscosity and fluidity of mold flux can be improved by adjusting the component ratio in this iso-viscosity diagram for applications in the steel industry.