Failure of high-strength bolts at the flange connection of wind turbine affects the safe operation of the structure, taking a 4 MW monopile wind turbine of a wind farm in Rudong, Jiangsu Province, China, as a prototype, to establish a local flange structure containing threads and an overall wind turbine model considering the flange connection structure, to carry out a study on the change rule of frictional dissipation energy of the local flange structure under the eccentric load as well as its failure behavior under the ultimate state, and then to explore the relationship between the number of bolts loosened in different positions, the proportion and the structural load carrying capacity through the overall wind turbine model. The analysis results show that the sensitive loosening position of the local flange bolt structure is located in the first contact thread near the loading end, and the ultimate damage form is the fracture damage of the unspun part of the nut and the screw. For the integral flange model, structural capacity increase with the number of bolts loosened on the tension side, showing a trend of decreasing and then levelling off, and the amount of flange separation is sensitive to the initial loosening of the bolts on the tension side.