Abstract: The defects of alarm steam generator heat transfer tubes during ultrasonic examination were counted and analyzed. There were mainly 5 typical types of defects. The surface macroscopic observation and defect depth measurement are carried out by means of metallographic microscope to determine the cause of the defects. The results show that the main defects during the actual production process of steam generator heat transfer tubes are longitudinal defects. The transverse defects are rare and most of them are external surface defects. The depth of the defect is not linearly related to the maximum ultrasonic amplitude of the defect.