Abstract: This paper applies magnetic flux leakage testing principle to detect the micro cracks on serving rail surface and sub surface by using high-speed flaw detector. The test method uses the rail surface adaptive and bearing mechanism, in which there are hall array sensors on the bottom of the mechanism and the mechanism enables the probes to cling closely to the rail surface. This method reduces the influence caused by the too large distances between the rail surface and the sensor and the shake of the train, and it has good effect on the detection of the outside seriously damaged rail defect. In this paper, the detection method is used to improve the accuracy of the existing rail surface defect detection method, which has great significance for the train safety.