Abstract: In order to measure actual effective length of bolt, the propagation law of high frequency ultrasonic guided wave in anchored bolt and free bolt was obtai ned. A frequency scanning was conducted on different anchoring bolt specimens within 1 ~ 15 MHz frequency scope.A relationship curve between the group speed and frequency was obtained from the experiment results. The results showed the speeds of high- frequency ultrasonic guided waves in 2.007 ~ 2. 312 MHz and 3.266 ~ 3. 491 MHz were almost identical when spreading in rock bolts of different anchoring quality. That within some frequency scope, the bolt anchoring quality would have a le ss influence to the propagation speed of the high frequency ultrasonic guided wave. Thus within the frequency scope, the propagation speed of the ultrasonic guided wa ve in the free bolt could be detected and a length of the anchored bolt with the same material could be calculated.