Abstract: In the field of high-speed rail transport, nondestructive testing of rail defects is essential to ensure the safety of railway traffic. The existing nondestructive testing technology can basically achieve the crack location, but there are still no effective detection methods for the characteristics of the crack, such as length, depth, extension angle, etc. In this paper, the differential eddy current testing system is used to obtain the amplitude and phase change of crack detection signals. Through a large number of experiments, defect samples with different length and depth are detected and analyzed. The relationship between the detection signal change and the crack characteristics is obtained. Furthermore, the relationship between the excitation frequency and the ability to resolve the defect depth is studied, and the influence of the probe diameter on the resolution of the defect length is investigated.