Abstract: Aiming at the serious damage of rail base metal from a railway site, means such as macroscopic examination, scanning electron microscope (SEM) fracture analysis and energy spectrum analysis, metallographic analysis, hardness testing and so on, were used to investigate the causes for the serious damage of the rail base metal. The results show that the metallurgical defect in the rail was the reason for the serious damage of the ultrasonic flaw detection, and the metallurgical defect was mainly related to the existence of cracks in the rail slab. The high-carbon martensite of the defect position of the rail was related to the rapid cooling speed, and the widmanstatten was due to the overheating when rolling. Steel mills should be recommended to strengthen the appearance inspection of slabs and ultrasonic flaw detection of finished rails to improve rail quality and stability.