Abstract: Preferred fluorine-free pre-melting slag composition was attained by analyzing the slag viscosity and melting point of impact factors and orthogonal experiments (CaO=48 %, Al₂O₃=40 %, SiO₂=4 %, MgO=8 %) to avoid the fluorine pollution based on the molten slag theory. The viscosity and melting point were 0.78 Pa·s and 1389 ℃. The industrial tests showed that the slag quickly formed at the beginning of refining, which established the foundation to accelerate the production pace. The slag, compared with the slag with CaF₂, showed no pollution of fluorine. The average desulfurization rate of steel was 86.5 % with an addition by 7.5 %. The average contents of N, H, O in the billet were 47.5×10^-6, 2.4×10^-6, 32.5×10^-6 respectively, with reductions by 13 %, 15 % and 15 %. The billet inclusion size dropped significantly with improving billet.