Abstract: The viscosity of solid-containing slags can be calculated using the Roscoe equation combined with the calculation functions of multi-component & multi-phase equilibria and pure-liquid slag's viscosity in FactSage software. An FeO-SiO₂-Fe₃O₄-CaO-Al₂O₃-MgO system used in copper smelting was studied in this paper. Firstly, the parameters of the Roscoe equation were fitted using the measured viscosities of analogous slag in the literature, and the accuracy of viscosity calculated by the method for this slag system was verified additionally. Then, the effect of slag components' contents on the equilibrium phase-composition and viscosity was investigated based on the calculation model. Accordingly, the reasonable proportions of components in the slag were obtained. When the mass fractions of slag components are FeO 40%~60%, SiO₂ 25%~40%, Fe₃O₄ 0%~15%, CaO 0%~10%, Al₂O₃ 0%~8%, and MgO 0%~4%, the molten slag with good liquidity and fewer solids can be obtained in the smelting process.