Abstract: Under long-term operation, the wear between journal and sleeve of a ventilator drive shaft was severe. The journal surface was continued to be used after being repaired by surfacing welding, but fracture appeared soon after. Through macroscopic observation, chemical composition analysis, tensile test, hardness test, microstructure observation and fracture analysis and other methods, found out the reasons for ventilator shaft fracture. The results showed that ventilator shaft fractured with low-cycle (high stress) multi-source fatigue. A large number of tire tracks could be seen at ratchet step at the edge of fracture. The surface of repaired ventilator shaft shoulder had abnormal microstructure and hardness, and the unprocessed bead weld toe at reduced diameter aggravated stress concentration on shaft shoulder. Under the action of alternating load, the keyway and weld toe of weld bead became the source of fatigue cracks, and rapidly expand until the fan shaft fractured under working load.