Abstract: The fracture occurred at the shaft body of 20CrNiMo steel pin shaft in service. The fracture reasons were analyzed by chemical composition analysis, fracture analysis, metallographic examination and hardness test. The results show that the fracture of the pin shaft was a multi-source fatigue fracture under the condition of stress concentration, after laser marking, the outer surface of the pin shaft produced secondary quenching structure, secondary tempering structure and crystal cracks, under the combined action of installation and service load, the pin shaft body propagated along the crystal crack till fatigue fracture.