Abstract: To reduce the number density of inclusions in SPHC steel produced by the direct-up process after leaving the argon station,this paper combines thermodynamic calculations of the equilibrium iso-oxygen content,sulfur distribution ratio，ΔC/η diagram（■）,and slag phase primary crystallization zone diagram for the CaO-SiO₂-Al₂O₃-5%MgO-5%FeO five-component refining slag system in equilibrium with SPHC steel.Laboratory and industrial tests are conducted for verification.The optimized range of refining slag is controlled as follows:42%<w（CaO）<54%,w（SiO₂）<20%,18%<w（Al₂O₃）<40%,basicity R>3,w（MgO）is approximately 5%,and w（FeO）is approximately 5%.The experimental results show that the typical composition of 51.3%CaO-5.4%SiO₂-33.3%Al₂O₃-5%MgO-5%FeO slag system is suitable for producing SPHC steel by the direct-up process,which can control the inclusions in steel to around 50 inclusions/mm².