Abstract: After the 3 MW wind turbine was put into operation, the blade connecting bolts were broken many times. The fracture causes of bolts were studied by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, data comparison and verification, and boarding site inspection. The results show that the bolts suffered fatigue fracture due to periodic alternating stress. The assembly accuracy of bolts was insufficient, and there was eccentric phenomenon in the installation process, which led to the close contact between the bolt rod and the inner wall of the bolt hole, and the local extrusion force was too large. During installation, the application of lubricating paste was not controlled in place, resulting in uneven preload of blade connecting bolts, which eventually led to fatigue fracture of bolts. It was recommended to replace the damaged blade connecting high-strength bolts, strictly comply with the assembly requirements of high-strength bolts, and correctly apply bolt lubricating paste to ensure the safe and stable operation of the turbine.