Through scanning electron microscopy, the fracture morphology, graphite morphology and microstructure of the nodular cast iron QT5007 air compressor shaft were observed. The results indicated that the fracture mode of this shaft belonged to fatigue fracture, and such defects as abnormal nodularizing, floating of graphite and the coarse grains were the important factors causing the abnormal fracture. In addition, the stress concentration near the keyway point and thread root could also initiate the multisource crack in service, leading to the brittle fracture failure. Therefore, this paper proposed that preventive measures should be strengthened such as furnace examination and metallographic analysis, reasonable shaft structure and avoiding cutting thread in high load area on shaft to ensure the quality of nodular cast iron shaft.