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The detection sensitivity and magnetic flux leakage are decided by the magnetization of rail for fast magnetic flux leakage testing. The main work of this thesis is to make sure how speed of detection, excitation and lift-off shall affect the magnetization of rail. The magnetic field intensity between rail and yoke , amplitude of magnetic flux leakage signals and alteration of rail’s magnetic permittivity are analyzed, and it is found that increment of speed of detection makes for detection sensitivity when the speed is from 2m/s to 55m/s. Meanwhile, increase of excitation and decrease of lift-off enhance magnetization of rail, which is propitious to detection sensitivity.