Abstract: The viscosities of different slags in CaO-Al2O3-CaF2-SiO2 system were measured by internal rotating cylinder method involving a spindle, and the high-temperature slag phases were analyzed by XRD. Then the viscous flow activation energies of the different slag samples were calculated. The results indicated that when the ratio of w (CaO)/w (Al2O3) is constant and the mass fraction of SiO2 in the slag system is lower than 8%, there is no obvious effect on the high-temperature viscosities of the slag samples. The viscosities of all the slags are lower than 0.5 Pa·s while the temperature is above 1490°C. When the mass fraction of SiO2 in slag comes up to 10%, the high-temperature viscosities of slags begin to decrease apparently. The viscosities of all the slags are lower than 0.2 Pa·s while the temperature is above 1440°C. The viscosities of slags decrease greatly with increasing SiO2 content because the original large meshed structure of slags is destroyed so as to reduce obviously the slag basicity. The viscous flow activation energies of the slag system CaO-Al2O3-CaF2-SiO2 tend to such a change that is compatible with the change in measured viscosities of slag samples.