Abstract: The slag detection system of hot metal is a set of slag grading system based on machine vision. Its main function is using a high resolution camera to observe the real-time situation in the molten iron tank, imaging through the industrial control comput-er and analyzing the thickness and the range of iron slag in the obtained image, judging the level of slag according to the standard data-base. This system can improve the automation level of the grade evaluation of online slag detection, further, not only reduce the influ-ence of the unstable factors of the workers' experience but realize the workers labor protection as well.