The structure health monitoring of high voltage transmission tower was very important to ensure the safe operation of the transmission line. When the transmission tower structure was damaged, the gusset plate bolts would be loosened first. The multiscale model of the transmission tower structure is established by using ABAQUS finite element analysis software. The energy change rate of the wavelet packet was taken as the damage index. The damage identification was carried out under different working conditions when bolt looseness of a joint plate at the tower leg. The signal transmission equipment was a 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 the gusset plate. It is also sensitive to bolt looseness near the measuring point.