The reasonable content of MgO in refining slag calculated by the thermodynamic calculation software FactSage ranges from 4% to 8%, in which 6% is the best. In addition, slag-steel equilibrium at 1873K between SiO2-CaO-Al2O3-6%MgO quasi-ternary slag and liquid steel were analyzed on the basis of results of industrial sampling combined that calculated by FactSage both in liquid region and solid-liquid coexisting region with CaO saturation. It indicates that the refining slag with high basicity and high w(CaO)/w(Al2O3) (C/A) is favorable for control of low oxygen, low sulfur and low silicon. However, slag with too much CaO is disadvantageous of deoxidation and desiliconization that suggests refining slag is not the whiter the better as the supersaturated CaO do not take part in the slag-steel reaction and even worsen its kinetic conditions. Furthermore, the optimized compositions of refining slag in LF for SPCC in Jiuquan Iron and Steel Corporation (JISCO) are: CaO 50%-55%, Al2O3 30%-36%, SiO2 1%-6%, MgO 4%-8% and 6% is best, basicity 9.0-14.0 and w(CaO)/w(Al2O3) is 1.5-1.8. Finally, both slag-steel equilibrium experiments in laboratory scale and industrial trials in plant scale verified that optimized slag has a better ability for deoxidation, desulfurization and control of silicon content, and could better control inclusion composition as well as its removal.