Abstract: The foaming indexes of a group of refining slag were measured. The refining slag with better foaming ability was chosen,its composition (mass frachon in %) is CaO, 53.25, SiO₂, 17.75, MgO, 9; Al₂O₃, 15 and CaF₂, 5. The relationship between slag foaming index and physical properties of the slag was obtained by dimensional analysis, and the expression indicates that viscosity of slag is the most important factor which influences foaming index. The influence sequence of slap composition on foaming index was also obtained as follows: CaF₂→MgO→Al₂O₃→ B (CaO/SiO₂).