The test of desulphurizing by CaO-SiO

2

-MgO-Al

2

0

3

refining slag with basicity R (CaO/SiO

2

) 2-7 in a medium frequency induction furnace has been carried out by quadratic orthogonal regression test scheme to establish the mathematical model of relation between ingredient of slag and steel-slag sulfur partition ratio Lfi and examine the effect of slag basicity

slag index MI (R ： Al

2

0

3

)

content of CaF

2

MgO and FeO on sulfur partition ratio Ls. Results showed that the desulphurizing efficiency of slag with basicity R 3.5 ~5. 0 and slag index MI 0. 25 ~0.40 was better； and the optimum ingredient of refining slag was 9% CaF

2

8% MgO

13%Al

2

O

3

<

0. 5% FeO with R =4.