Abstract: Slag-making monitored control system which based on sonic level provided a useful method for steelmaking in converter. According to the sonic level in converter, the system can decide the optimum slag-making region autornatically, and show the lance hight and the information of fluxes charging in ordor to guide steelmaking operation. 5 to 10 seconds before slag overflow or slag over-viscous, the system can give an advance warning with the hitting ratio of 86.7% for slag overflow and 94.6% for slag over-viscous. In addition, the system has the function of predicting the content of. phosphorus in bath.