A sulphide capacity prediction model of CaO-SiO2-MgO-Al2O3 slags has been developed based on the ion and molecule coexistence theory(IMCT). Sulphide capacity(Cs) of slag for blast furnace (BF) with high Al2O3 in the CaO-SiO2-MgO-Al2O3 system at 1773K were measured by applying slag-metal equilibrium method. The feasibility of the developed IMCT model is verified by the sulphide capacity measured in the experiment. Effects of R(w(CaO)/w(SiO2)), w(MgO)/w(Al2O3) and w(Al2O3) on sulphide capacity were discussed. There is a good linear relationship between the experimental value and the predicted value. Therefore, the theoretical model of ion and molecule coexistence can be used to calculate the sulphide capacity of CaO-SiO2-MgO-Al2O3 quaternary slag system. When w(Al2O3)=20% and w(MgO)/w(Al2O3)=0.5, the sulphide capacity of slag increased with the increase of R. When w(Al2O3)=20% and R=1.30, the sulphide capacity of slag increased with the increase of w(MgO)/w(Al2O3). When R=1.30 and w(MgO)/w(Al2O3)=0.4, the sulphide capacity of slag decreased with the increase of w(Al2O3).