Abstract: Rapid relative motion between the detection device with the DC exciter magnetic yoke and measured rail track impacts on detection signal under the condition of Magnetic Flux Leakage (MFL) detection for rail crack defects at high speed. This paper investigates the velocity effect of magnetization in high speed MFL detection from three aspects, including theoretical model analysis, finite element simulation and high-speed detection experiment. Improving the detection speed will shorten the time of magnetization process and weaken the material magnetization intensity during the high speed MFL detection. Movement of the magnetic field will produce eddy current in the rail track. Research results show that the decrease of material magnetization intensity is the main influential factor for detection signal, compared with the eddy current effect. Within the scope of 200 km/h, MFL method can realize the effective detection for rail crack defects, and improving the detection speed results in the increase of MFL detection signal amplitude.