Considering the importance of absolute displacement monitoring and the limitation of normal displacement monitoring

this paper develops a FBG displacement sensor with temperature self-compensation to measure the inner absolute displacement in the rock.This sensor uses the FBG as key sensitive components.By calibrating this new sensor

the data show that the sensitivity of this sensor is 0.267 nm/mm

while the linear correlation coefficient is up to 0.99.We construct a FBG displacement monitoring system and use it in the model experiment of undersea tunnel.We detect the displacement change at nine key points in tunnel surrounding rock during excavation.Based on the analysis of experimental results

it is concluded that the changes of the displacement are less than 25 μm.This conclusion proves that this sensor meets the demands of the experiment and has perfect practicability.