Abstract: The fracture reasons of a 35CrMo steel desulfurization and oxidation fan shaft in service were carried out by means of metallographic examination, chemical compositions analysis, mechanical test combined with fatigue strength intensity checking and actual conditions searching. The results show that fatigue cracks produced at the stress concentrated keyway which first expanded along the axial direction, then along the cross-sectional direction, and eventually resulted in the transversal low stress and high cycle fatigue fracture. The mechanical properties of the fan shaft met with the requirement of national standard. According to the fatigue strength intensity checking results, under the normal load, the fan shaft was safe. The desulfurization and oxidation fan worked in harsh environment, vibration and long time overload operation were the fundamental reasons for early fracture of the fan shaft.