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Summary Excessive subgrade permanent deformation resulting from heavy vehicle traffic and freezing–thawing cycles is one of the primary causes of road damage in seasonally frozen regions. In this paper, an implanted cantilever sensing beam based on fiber Bragg gratings (FBGs) is proposed for subgrade deformation monitoring. Two optical fibers with inscribed FBGs were adhered to the opposite external sides of a PVC pipe which was selected as substrate. After laboratory calibration tests, the cantilever sensing beam was installed horizontally through the subgrade of a testing section. The beam will be bended driven by the deformation of the soils under the action of vehicles, and the deformation is recorded by the FBGs and calculated through principles of continuum mechanics. The subgrade deformation is considered to be the deflection of the sensing beam. The feasibility of the developed method was validated based on the results of field testing conducted for a three‐year period. It has been found that the combination of freezing–thawing cycles with heavy vehicle loading magnifies the extent of permanent subgrade deformation. The developed FBG‐based implanted cantilever sensing beams are thereby demonstrated to provide a feasible and effective approach for the in situ monitoring of subgrade deformation.