Abstract: The normal service life of the driving system main shaft of a construction machinery is half a year, but it fractured after 70 days. The fracture causes of the main shaft were analyzed by means of macro analysis, chemical composition analysis, metallographic examination, hardness test, micro analysis and energy spectrum analysis. The results show that the fracture mode of the main shaft was fatigue fracture. The eccentric wear of the main shaft and the shaft sleeve caused serious wear on the main shaft surface. The temperature rise caused by wear was higher than austenitizing temperature, and the white bright area of local untempered martensite was formed after cooling. The cracks in the tempering area around the white bright area became the fatigue source. During the high speed rotation of the main shaft, the cracks propagated until the fatigue fracture occurred.