As the strain sensing element of structural health monitoring, the study and application of the fibre-optic Bragg grating (FBG) has been widely accepted. The accuracy of the FBG sensor is highly dependent on the physical and mechanical properties among the bare optical fibre, protective coating, adhesive layer and host material, namely, the strain interface transferring characteristics among layers. This paper takes the embedded FBG sensor as an object of study. Firstly, the general expression of multiplayer interface strain transferring mechanism is derived. Secondly, based on the defined average strain, the error modified equation of FBG sensor is obtained. Finally, in the light of embedded tube-packaged FBG and FRP-OFBG strain sensors, which developed by ourself, the corresponding strain transferring error, laws were studied, and the corresponding error modification coefficients were given too, which were validated by experiments. The achievements provide theories for the development and application of the embedded FBG sensors.