Thermodynamic study on CaO-SiO2-Al2O3 slag system was carried out to understand the refining ability of Ca and Al in ferrosilicon. The effect of slag composition and oxygen partial pressure on refining ability of slag was also thermodynamically evaluated. Experimental results indicated that equilibrium concentrations of Ca and Al are both decreased with increasing SiO2 activity of slag and oxygen partial pressure. Also, the effect of activity coefficients of Ca and Al in molten alloy was not significant, while activities of each component in slag were important determining factors. Refining ability of Ca and Al of CaO-SiO2-Al2O3 slag from ferrosilicon is possibly increased by controlling interfacial oxygen partial pressure and activities of components of slags.