Abstract: A wind turbine tower collapse occurred when working in a wind farm. Through the investigation of the site of the wind turbine collapse, and the analysis of operating data, fault records and maintenance records, it is considered that there were welding defects in the seam between the bottom of the second section of the tower and the flange. The defects expanded and cracks appeared under the operating load of wind turbine. Due to the continued expansion of the cracks, the strength of the fracture of the tower was not enough to withstand the external force, resulting in the break of the tower and the collapse of the wind turbine. In view of the problems existing in the investigation of the manufacturing defects, operation and maintenance of the wind turbine, some suggestions and preventive measures for the subsequent monitoring operation and maintenance work are put forward.