As the package structures of a sensor seriously interfere the strain responses measured by a Fiber-optic Bragg Grating(FBG), this paper focuses on the relationship of measured strain and true strain in the actual measurement. It establishes a strain transfer function for embedded FBG sensors and verifies the validity of the transfer function and the influence of different parameters on the measured strain. Firstly, based on mechanical characteristics of embedded FBG sensors, the shear stress distribution with a form of polynomial is presented, then the strain transfer function is established and verified by taking a numerical method and an experiment. Finally the influence of sensor length, cementation layer modulus and cementation layer thickness on the measured strain is analyzed. Experimental results indicate that the strain transfer function is valid. Moreover, the thinner the cementation thickness and the higner the cementation modulus are, the more convenient the strain transfer is. The strain transfer function satisfies the accuracy requirement of embedded FBG sensors because the calculation error is controlled within 5%, which is considered as a guidance for its practical application.