Abstract: Rapid recognition of unstable rocks has been one of the hottest issues in the study of geological hazards for many years.Essentially, the cracking of rock bridge is an important factor causing rock mass instability.Based on rock bridge length, the devastating unstable rocks can be quickly found out.However, it is very difficult to directly obtain the rock bridge length because it requires not only detailed geological information, but also a large number of in-situ experiments.These tasks are often time-consuming and expensive, so it is easier to apply an indirect evaluation method in engineering practice.In this study, using a Laser Doppler Vibrometer, the natural vibration frequency is obtained by remote vibration monitoring.Then according to physical model experiment, the indirect measuring method of rock bridge length based on vibration frequency is analyzed.Finally, the fuzzy recognition algorithms for the unconformity rate of rock bridge and the errors are discussed as well.The results show that the natural vibration frequency monitoring based on Laser Doppler Vibrometer can provide data support for rock bridge length.The maximum error is 16.47%, the minimum is 0.46%, the average error is 6.81% and the correlation is as high as 0.985 7.Meanwhile, the technology can also realize the fuzzy recognition of rock connectivity rate, and the average error is only 3.25%.This fuzzy recognition algorithm does not need to know the size and quality of blocks and other information, and provides a relatively convenient index to achieve the purpose of rapid identification and early warning of unstable rocks.The vibration monitoring based on LDV can provide important data support for a better response to rock collapse in engineering.