Abstract: In the process of molten steel casting, outflow slag must be detected when liquid steel in a ladle flowing into a tundish. After lots of investigations on the slag detection technology and experiments in the Steelworks internal and external, a kind of high-sensitive slag detection system has been put forward. It alters the traditional single detective mode in hardware, meanwhile makes the software of feature extraction simple. The results testing in the Steelworks proved the effectiveness of the slag detection system, which can control the slag flowing into a tundish and create benefit for Steelworks evidently.