Abstract: Low frequency ultrasonic longitudinal guided waves were applied to inspect the anchorage quality of rock bolts. The effects of modulation wave type and signal's cycle number on signal spectrum were firstly analyzed, and sinusoidal waves modulated by Hanning window were selected for guided wave testing. Then longitudinal guided waves with frequencies 20~100 kHz reflected from the upper interface of grouted rock bolts were numerically simulated. Based on theoretical analysis and experimental investigation, a new method was proposed so that the debond length of grouted rockbolts could be determined by the time difference between bolt end reflected wave and upper interface reflected wave, and then the bonding compactness of a grouted rock bolt was calculated. The results show that guided wave reflection coefficient from the upper interface of grouting rock bolts decreases with the increase of frequency. The experimental results of guided wave reflection coefficient vary considerably from simulation ones, which is due to the transducer's resonant frequency and the energy loss between the transducer and the bolt end. The testing results of bonding compactness agree well with theoretical results.