The sulfide capacity (CS) and equilibrium distribution ratio of sulphur (LS) are important indicators to measure desulfurization capacity of slag system. The sulfide capacities of CaO-SiO2-MgO-Al2O3-FetO slags are calculated by optical basicity model and KTH model, and compared with measured values in literature. The results show that the values calculated by KTH model are nearer to measured values than by optical basicity model, so the former can be used to predict CS of different slag compositions. The factors affecting CS are also studied in detail. The results show that CS increases with increasing slag basicity and temperature and decreases with increasing MgO and Al2O3 contents in slag.