To analyze whether the combination of flexible materials with low Young′s moduli and rigid silica with a high Young′s modulus produces any practical problems, such as creep or strain transfer differences caused by rigid-flexible strain coupling when the fiber Bragg grating(FBG) senses shape deformation, four different Young′s modulus soft matrices were prepared by using silica gel (commonly used in soft robots) and polydimethylsiloxane (PDMS). Three FBGs were implanted in each soft matrix to form four flexible sensors with shape measurement capabilities and subjected to bending tests. The consistency between the experimental results and the theoretical derivation was verified theoretically via the strain transfer model.The results show that there is a creep-slip problem caused by rigid-flexible coupling when the soft matrix and FBG are combined, with the wavelength drift tending to be couple stable after about 30 minutes. Following creep stabilization, the wavelength drift of the three FBGs in the four flexible sensors shows good linearity and consistency. In addition, the larger the rigid-flexible difference between the fiber and the substrate, the more severe the coupling creep and the smaller the strain transfer rate. The maximum and minimum strain transfer rate are 0.680 and 0.260, respectively, while the maximum and minimum sensitivities are 56.649 and 35.668, respectively. These results provide a scientific reference for research focusing on the shape measurement technology of soft robots using implanted FBGs.