Abstract: Bolt as an important connection component of the infrastructure structure, its reliability influences the security of overall structure, so it is necessary to monitor the health of the bolt connection. Bolt looseness and preload degeneration are the most common types of damage to bolts. Due to the advantages of being sensitive to micro-damage and small energy attenuation with long propagation in the structure, the ultrasonic guided wave was widely used to evaluate the health state of structure. In this paper, ultrasonic guided wave was used as a damage detection method and the bolt pre-tightening force was characterized by the energy index of signals obtained from wavelet packet decomposition and the feasibility was verified by experiment.