Abstract: Given the theory governing the molecular ionic structure of slag, a model was determined for calculating activity of an eight-element slag system consisting of CaO-SiO2- Al2O3-FeO-CaF2- La2O3-Nb2O5- TiO2. The activity change of components containing niobium, rare earth elements, or titanium was calculated and analyzed. Activity calculated by the model conformed well to experimental data. The component containing niobium were mainly FeO · Nb2O5, those containing lanthanum were mainly La2O3 and La2O3 · Al2O3, and the components containing titanium were mainly 2FeO · TiO2 and FeO · TiO2. As FeO mass fraction in the slag decreased, the niobium component changed from FeO · Nb2O5 to CaO · Nb2O5, while the titanium component changed from 2FeO · TiO2 and FeO · TiO2 to TiO2. w(CaO)/w(SiO2) was an important factor influencing activity of the components containing niobium or titanium at low levels of FeO mass fraction. Activity of 2CaO · Nb2O5 and CaO · TiO2 markedly increased with the increasing w(CaO)/w(SiO2).