To discuss the relative slipping at the interface between a flexibly embedded fiber Bragg grating sensor and a substrate, the strain transfer was derived for an ideal case between the materials of an embedded fiber sensor. ANSYS software was used to establish a simulation model and to analyze the effects of the axial tensile force and the semi-embedded length and encapsulation substrate for the axial strain relative errors with and without relative slipping. The results of the numerical simulations show that the relative strain errors are smaller at the ends of the fibers and larger in the middle for the same tensile force, indicating that the strain transfer effect is location dependent and that the choice of a semi-embedded length of the fibers greater than 40 mm helps to reduce the relative errors. Meanwhile, five flexible sensors with different half-embedding lengths were experimentally encapsulated and subjected to axial tension-strain experiments, which showed the best strain transfer at a half-embedding length of 60 mm, and the experimental results were consistent with the numerical simulation results. The experimental results provide some theoretical and experimental basis for parameter optimization of flexible fiber grating sensors.