The structure health monitoring of high voltage transmission tower was very important to ensure the safe operation of transmission line. When the transmission tower structure was damaged, the gusset plate bolts would be loosened first. The multiscale model of transmission tower structure is established by using ABAQUS finite element analysis software. The energy change rate of wavelet packet was taken as damage index. Damage identification was carried out in different working conditions when bolt looseness of a joint plate at tower leg. The signal transmission equipment was sensor based on Internet of Things technology. The data acquisition equipment was PVDF piezoelectric film strain gauge. Applying excitation loads at the bottom of the actual transmission tower structure, The dynamic strain of the measured points on the gusset plate was extracted to verify the damage identification method. The test results show that the damage index can accurately identify the damage degree of gusset plate. It is also sensitive to bolt looseness near the measuring point.