Abstract: Aiming at the problem of discarding defects found in a heavy turbine shaft forging of 20SiMn steel produced by a factory during the ultrasonic flaw detection, the unqualified part was located and dissected, and means such as acid leaching macro-structure test, scanning electron microscope fracture analysis, energy spectrum analysis and metallographic analysis were used to find out the cause of the ultrasonic flaw detection disqualification of the shaft. The results show that the high-level general loose defects in the shaft forging caused the ultrasonic sound energy to be greatly attenuated during the inspection, which made it impossible to find out the quality of the shaft center. In addition, the sulfide inclusion center accumulation in the shaft caused the formation of crack sources during forging deformation and resulted in the center crack defects.