Abstract: Based on the assumptions such as plane section assumption, the mechanical model of flange bolt group is established upon local bolt loosening. By introducing the concept of influence line, a method is proposed for detecting local bolt loosening in the main wind direction of wind turbine tower flange based on the influence line of flexibility coefficient. The location identification and degree evaluation of local bolt loosening in main wind direction can be realized by drawing the curve of nose azimuth-flexibility coefficient. The finite element analysis model of wind turbine tower flange bolt group is established to verify the validity of the detection method. The strain gauges are arranged in the orthogonal directions at the joint of the connecting flange of the tower cylinder of the wind turbine and connected to the SCADA system of the wind turbine, which is collected synchronously with the upper unit parameters such as wind direction, angle and hub speed. The on-line analysis program is developed, and the on-line monitoring and evaluation method of wind turbine tower flange bolt looseness identification based on influence line is put forward, which can be used in practical projects.