Abstract: Based on the stress characteristics of bolts, it is proposed that the bolt group with pre-tension loss under external force experiences two stress states, namely normal working state and loosening state. In the loosening state, the external force is completely borne by the bolt, and the connection stiffness of the flange will decrease significantly with the increase of the number of bolts under loosening state, resulting in a significant change in the strain of the tower barrel near the connection surface. In view of the large number of bolts of the connecting flange of the wind turbine tower cylinder and the difficulty in monitoring, Saint-Venant principle was introduced to put forward a field monitoring scheme for monitoring the strain on the upper and lower walls of the cylinder above the bolt group. In other words, by determining the difference of the departure point and height h of the strain ratio K-value curve at the upper and lower measuring points, the degree of loosening of the bolt group could be accurately identified. The results of finite element analysis show that when the distance between the upper and lower measuring points is greater than 0.7d (d is the diameter of the tower barrel), the strain at the measuring point above the diameter of the tower barrel is only related to the load, and has nothing to do with the loosening degree of the bolt group. It is not only feasible to regard it as “equivalent load”, but also can change simultaneously with the strain at the lower measuring point. At the same time, in order to improve the accuracy and economy of monitoring the loosening degree of bolt groups, it is recommended to adopt the dislocation arrangement of the upper and lower measuring points, which can not only reduce the number of monitoring points effectively, but also meet the monitoring accuracy and economy requirements, and can be selected for practical projects.