Abstract: The fiber optic sensing technique has been widely applied to geohazard monitoring and deformation measurement of geomaterials. This paper aims to understand the coupling mechanism between strain sensing optic fibers and the surrounding soil masses for interpreting the monitoring results. It examines the influence of fiber embedment length on the mechanical property of fiber-soil interface. There are insufficient investigations on the strain distribution and transfer mechanism along the sensing optic fiber during pullout process. Using the newly designed pullout devices, this paper evaluates the influence of four different embedment lengths of sensing optic fiber on the fiber-soil interfacial property. A mathematical model is used to fit the pullout curves. The results show that the effective pullout displacement and the maximum pullout force increase linearly with the increase of fiber embedment length. But the fiber-soil interfacial shear strength decreases with the increase of the embedment length. Furthermore, the Brillouin optical fiber time domain analysis(BOTDA)technology is used to measure the strain distribution along the sensing optical fiber during its gradual decoupling process from the soil mass. The distribution of fiber-soil interfacial shear stress is also calculated, which shows that the interface failure is highly progressive. These findings can provide reference for understanding the interaction mechanism between strain sensing fiber and surrounding soil, and promote the application of fiber optic sensing technique to geoengineering monitoring.