Abstract: Phased array ultrasonic technology was used to detect rail transverse defects. First, the rail transverse defect detection model was established and analyzed. It is found that refraction angle, the inclination angle and transverse position of the probe on rail surface are the main factors that affecting the defect detection. Then, by combining the theoretical calculation and simulation analysis, the optimal parameters of phased array probe for rail transverse defect detection were obtained. Finally, based on the parameters above, the phased array probe was designed and tested on various types of damages on 60 rail and 75 rail respectively. The experimental results show that the signal-to-noise ratio of the phased array ultrasonic probe for all defect detection is greater than 12 dB, that is, the rail transverse defect detection method based on the phased array probe can adapt to the changes of the heights of different rail types and the change of rail jaw angle, and the rail head area was fully covered by ultrasonic.