Abstract: The shaft of external supply pump was fractured in use. The fracture reason was analyzed by means of macroscopic and microscopic examination, chemical composition analysis, hardness testing and so on. The results showed that the main fracture reason was the improper heat treatment process, which reduced the mechanical properties of shaft. Furthermore, the fatigue cracks initiated in the root of keyway which was stress concentration position, and the cracks extended further and further, the shaft was fractured at last.