Abstract: At present, the main methods used are manual detection and ultrasonic testing, and the width and depth of the cracks of the rails mainly detected are mostly millimeters, and there is no detection method for the micron-scale rail cracks with very small openings and shallow depths. In view of the current shortcomings, this paper proposed a rail crack detection method based on diamond nitrogen-vacancy (NV) color center, and developed a corresponding magnetic flux leakage-quantum detection device. In order to detect the cracks on the surface of the rail, the permanent magnet iron was used to magnetize the rail surface, and the magnetic flux leakage signal of the crack on the rail surface was detected by the high sensitivity and high resolution characteristics of the NV color center to the magnetism. The manganese steel rail was selected, the transverse cracks were designed and manually processed on the surface, and the detection experiment was carried out with the designed rail flaw detection vehicle. The experimental results showed that the measurement sensitivity of the experimental device was 7 $\text{nT}/\sqrt{\text{Hz}}$, and the crack detection on the rail surface could be completed with a minimum width of 100 μm and a depth of 100 μm.