Addressing the challenges of low sensitivity and inadequate spatial resolution in conventional deep displacement monitoring methods, a novel approach is introduced that involves the surface mounting of ultra-weak fiber Bragg grating (UW-FBG) fiber optic cables to the inclined tube for monitoring deep displacements of landslides. The UW-FBG strain sensing principle is theoretically analysed, and the calculation formula for wavelength shift in relation to deflection and displacement is derived. The strain characteristics of the UW-FBG fiber optic cable are calibrated in the laboratory, and the performance of the UW-FBG fiber optic cable, when applied to the surface of an inclinometer tube, is experimentally validated. This technology is then implemented for monitoring landslides on the banks of the Three Gorges region. The findings indicate that the deflection values obtained from the UW-FBG inclinometer structure closely align with the actual deflection values measured by a micrometer, with an error margin of approximately 0.5 μm and a transfer efficiency of 95.51%. This method accurately captures deep displacement during the rainy season in landslide monitoring, offering a highly promising solution for the surveillance of deep displacement.