Abstract: The borehole full-section optical fiber monitoring technology has been widely applied in the monitoring of geological disasters such as ground subsidence and mine collapse. The strain coupling between the borehole backfill and the directly buried sensing cable is the key to the influence of the accuracy of optical fiber monitoring results. The strain coupling between the fiber optic cable and the backfill of sand with different particle sizes (0.5~4 mm) under the confining pressures of 0~1.0 MPa is investigated by using the coupling analysis device of controllable confining pressure cable and backfill. The results show that under the same confining pressure and pull-out displacement, the strain coupling between the optical cable and the sand decreases while the particle size increases, that is, the coupling between the sand of 0.5~1 mm and the strain sensing optical cable is the strongest. Using 10000 με as the maximum strain monitoring range of sensing cable, the strain coupling between the sensing cable and the sand soil is evaluated. When the deformation coordination coefficient of the fiber optic cable and the sand reaches 0.9, the critical confining pressures with particle sizes of 0.5~1, 1~2, and 2~4 mm are 0.14, 0.33, and 0.52 MPa in order. The interface shear stress between the fiber optic cable and the sand first increases and then decreases. With the increase of the pull-out displacement, the peak value and the transfer depth of the interface shear stress increase. The results provide a scientific basis for determining the depth of the critical confining pressure of the borehole full-section optical fiber monitoring.