Abstract：Longitudinal guided wave is used to inspect the service load in the partially grouted bolt nondestructively. The stress distribution characteristics of free section and anchoring section in the partially grouted bolt under service load was first analyzed and tensile stress keeps constant in the free section of the bolt. Then group velocity of longitudinal guided wave propagating in the free section of the bolt under different tensile stresses was calculated. The results show that the group velocity decreases linearly with the increase of tensile stress within the elastic distortion range. Experiments were conducted to study the reflection of longitudinal guided wave from the upper interface of anchoring section of partially grouted bolt；and it shows that the upper interface reflection wave can be observed clearly when the frequencies of guided wave are 45–75 kHz. The reflection of longitudinal guided wave from the upper interface of partially grouted bolt and the time of guided wave propagating in the free section of the bolt can be used to determine the service load in the partially grouted bolt；and the theoretical and experimental results of service load-propagation time have the same trend.
