At present, free α-SiO₂ content is often determined by the pyrophosphate method and X-ray Diffraction (XRD) method. However, insoluble materials including FeO, Fe₃O₄ and Fe₂O₃ in mill scale cannot be decomposed by pyrophosphate, and thus the pyrophosphate method is not suitable for the determination of free α-SiO₂ content in mill scale. A method for detecting free α-SiO₂ content in mill scale through X-ray diffraction K value method has been developed. α-Al₂O₃ was used as the reference material. Diffraction peaks of crystal face (101) for α-SiO₂ and crystal face (012) for α-Al₂O₃ were used for measured diffraction peaks. Samples for detecting K value were prepared by mixing α-SiO₂ with α-Al₂O₃ at a mass ratio of 1:1. The acquired K value is 7.86. The measured K value has been applied to the determination of mill scale samples with known free α-SiO₂ content, and the analytical results of free α-SiO₂ content are in good agreement with practical values. Detection range of this method is that free α-SiO₂ are equal or greater than 0.5%. This method is convenient and rapid, and has a wide detection range and can meet the need of imported mill scale inspection and supervision work.