Abstract: The effect of CaO-Al₂O₃-SiO₂ slag ingredient on T[O] in 0. 42C-1.15Cr-0. 2Mo steel and sulphur partition ratio Ls- (S)/[S] at 1 873 K has been studied by experiment of MgO crucible in Si-Mo high temperature furnace. It was found that with increasing slag basicity- ( CaO + MgO)/SiO2 from 1 to 3 , the T[O] dramatically decreased from 0.001 4% to 0. 001 0% , and with further increasing basicity, the change of T[O] was not obvious； as ( FeO + MnO) W0. 50% , the T[O] was WO. 001 0% and as (FeO + MnO) more than 0. 50% , the T[O] obviously increased to 0.001 4% ； the slag basicity, activity of Al₂O₃ in slag and [ Al] had obvious influence on Ls, that was with increasing the activity of Al2O3 in slag, the Ls decreased and with increasing the slag basicity and [ Al] , the Ls increased.