Free cutting steel is energy save material gradually extensive applied in the development of the engineering industry, but it is one of the three steels hard to continuous casting. At high temperatures, the corrosion of refractories by free cutting steels is one of the reasons for free cutting steels hard to continuous cast with a long time. Through the observation of the used refractories microstructure and detail discussions, it is concluded that Al2O3 in alumina carbon materials for the continuous casting is easy to react with FeO or MnO in the steel to form the low-melting point material, which is the main reason for the corrosion of the refractories when used to cast free cutting steels. After decarburized layer is formed on the hot surface of the spinel-carbon material, low-melting point materials will infiltrate into the decarburized layer and separate the particles into an isolated state, which will result in the corrosion of the materials by the scour of the molten steel. Through the formation of composite bond in the spinel-carbon materials, the corrosion resistance to the free cutting steels can be further improved. Based on the corrosion mechanism, the suggestions for the functional refractories with high corrosion resistance to free cutting steels are also provided.