Abstract: With the increase of train speed and traffic density, high speed inspection for rail cracks is in great demand in these years, and the former flaw recognition methods based on static condition need to be improved. Based on the background mentioned above, the speed effect on rail magnetic flux leakage(MFL) inspection was researched. Firstly, the effect of the testing speed on the MFL signals was discussed theoretically by formulation; Secondly, the research model was established in Ansoft software by finite element method(FEM). The signal differences under various testing speeds were discussed, and some significant conclusions were summarized accordingly. Lastly, the influence rules of the testing speed on the characteristic signals were researched by the simulation examples of the depth and width recognition of the rail cracks.