The strain transfer of fiber Bragg grating (FBG) strain sensor determines its accuracy when monitoring the structural strain. To study the influencing factors of strain transfer of FBG strain sensor, the mechanical model of the sensor strain transfer is developed in this paper. The theoretical derivation of the strain transfer is conducted, and the formula of strain transfer rate of the sensor is obtained. Meanwhile, to ensure proper computation the finite element simulation and calculation are performed for its special structure. Furthermore, the influencing factors and effects of the strain transfer of the sensor are analyzed, and different influencing factors are simulated using the finite element simulation, which verifies the accuracy of the theoretical derivation.