Abstract: According to the need of controlling the slag content in the process of converter tapping, a slag detection system based on the principle of infrared temperature measurement was designed. Firstly the temperature of pouring stream was measured using the infrared temperature measurement system. The molten steel and slag were distinguished according to their infrared emissivities that had big difference in far infrared band and the real-time amount of slag contained was calculated. In different steelmaking sites(different atmospheric transmittances, different ambient temperatures, different tap temperatures, etc), the infrared emissivities of the molten steel and slag were changed, therefore a calibration method of the temperature threshold that was used to distinguish the molten steel and slag in a certain environment was proposed. A method that could quickly find the edge of the pouring stream was applied to complete the tracking of pouring stream, which could avoid the amount of slag was incapable of being calculated due to the change of converter angle(moving of pouring stream). Experimental results show that this method controls the amount of slag in the steel ladle and meet the requirement of molten steel refining.