By analyzing the virtual assembly technologies at home and abroad,it is found that due to the relatively large size of transmission tower components,the acquisition error of size parameters is relatively large,resulting in a relatively low accuracy rate of the application of virtual assembly technology in transmission towers.In this paper,a size detection system for internal threads in bolt holes is constructed,and the algorithm model is improved,which enhances the robustness of the model and improves the quality of reconstructed images.Through experiments,it is verified that the standard deviation of this size detection system is 0.000 854 mm and the range is 0.002 mm.Compared with the traditional size detection systems on transmission towers,it has a significant improvement in accuracy and stability,which has a positive significance for improving the accuracy of virtual assembly of transmission towers.