In this study, the effects of MgO content, Al₂O₃ content, TiO₂ content and C/S (=CaO/SiO₂) on slag flowability are investigated for blast furnace (BF) operation. The liquidus temperature and the viscosity of semi-synthetic slags are measured using an optical softening temperature detector and a viscometer, respectively. Based on the measured data, equations of the liquidus temperature and the viscosity for the semi-synthetic slags (SiO₂–Al₂O₃–CaO–MgO–TiO₂) were formulated via the multiple-regression method. In operation practice of BF, the slag flowability is mainly determined by both viscosity and liquidus temperature. A combined slag flowability index, including viscosity and liquidus temperature, has been designed in this research to indicate appropriate slag flowability. This index has been incorporated into the tapping system of CSC (China Steel Corporation) BF. Based on the calculated liquidus temperature and viscosity of the final slag inside BF, the system is able to predict the difficulty of slag flow for the coming tapping. It can immediately provide BF staffs some information to control the variation of slag flowability via the effective operation practice such as enforcing management of thermal state or adjusting the chemical composition of final slag via the burden of BF.