Abstract: The effect of refining slag on the total oxygen content and D class inclusions in SWRH82B steel was studied by laboratory slag/steel reaction equilibrium experiments; The formation conditions of MgO·Al₂O₃ spinel inclusions is explained from chemistries of molten steels and slag compositions by Fact-Sage software. The results show that low basicity and high Al₂O₃ content of refining slag is beneficial to the removal of D class inclusions; The melting point of primary style composite oxide inclusions become higher while basicity and Al₂O₃ content of refining slag is getting increase. Melting point get to lowest when basicity and Al₂O₃ content is 0.93 and 5.1% respectively; Compare with controlled trial, MgO·Al₂O₃ spinel inclusions exist in steel when basicity and Al₂O₃ content of slag is 1.14 and 25.6% respectively.