Abstract: The quantization of defect magnetic flux leakage signal is always an important direction in the field of magnetic flux leakage signal processing. In this paper, Fourier transform was applied to magnetic flux leakage signals. By the use of the amplitude spectral sequence of defect magnetic flux leakage signal, information entropy was extracted. The length and depth of defect were quantified through the combination of signal characteristic and BP network, and the quantization precision was estimated reach 88.48% and 84.17%. It was proved that information entropy was feasible in the quantization of defect magnetic flux leakage signal by theoretical analysis and experimental study.