The experiments on the foaminess of the CaO-MgO-Al₂O₃-SiO₂-CaF₂ refining slag system have been carried out. The obtained results are as followst (1) relative foaming height linearly increases with the increasing of the flowrate of blowing gas; (2) the proper content of MgO is about 11% when slag basicity B<2.5 and it should lower when slag basicity will be higher; (3) the better content of Al₂O₃, (MgO)+(Al₂O₃ ) is 15% and 20%~26% respectively at the range of lower basicities; (4) the effect of slag basicity on the foaminess is complex and its optimizing vaiue is 1.9 at the specific contents of MgO, Al₂O₃ and CaF₂.