To solve the problem that the failure of high-strength bolts at the flange connection of wind turbines affects the safe operation of the structure, the local flange model with threaded structure and the integral wind turbine model considering the flange connection structure are established, taken the 4 MW monopile wind turbine of a wind farm in Rudong, Jiangsu Province, China, as a prototype. The investigations are conducted on the varying pattern of the frictional dissipation energy of the local flange structure under eccentric load and the failure behaviour under extreme conditions. The relationship between the number and proportion of the bolts loosening at different locations with the structural load carrying capacity is explored through the overall wind turbine model. The results indicate that: the critical loosening point of the local flange bolt structure is situated in the initial contact thread near the loading end, and the ultimate failure mode is the fracture of the unscrewed portion of the nut and screw; for the integral flange model, the flange separation displacement exhibits a tendency to increase first and then level off with the number of bolts loosened on the tension side, which is more sensitive to the initial loosening of the bolts on the tension side.