Abstract: The high speed input shaft of tower crane was broken in the course of use. The causes of the fracture were analyzed by macroscopical examination, fracture analysis, metallographic analysis and chemical composition detection. The results show that the high speed input shaft is rotated bending fatigue fracture under low nominal stress and high stress concentration. The main reason for the fracture is the structural stress concentration at the concave corner of the step, which can reduce the fatigue strength of the shaft and promote the initiation of fatigue cracks.