Abstract: A main shaft fracture accident of wind turbines occurred in a wind power plant,resulting in varying degrees of damage to the rotor-regulating mechanism and gearbox in the hub.In this paper,the cause of the accident is analyzed,and it is determined that the cause of main shaft fracture is material fatigue damage under abnormal load.The detection method of main shaft cracks is put forward.At the same time,two technical reconstruction measures are taken,that is,installing temperature measuring device of main shaft bearing and on-line vibration monitoring instrument of units,which provides successful experience and operational guidance for monitoring and accident prevention of main shaft cracks of wind turbines.