Strain transfer ratio is one of the key characteristics to determine the accuracy of sensors for strain measurement and structural health monitoring. This paper presented a theoretical study on the strain transfer ratio of optical fiber sensors, which is generally bonded on the surface of target structure by adhesives to measure strain or stress. Compared to the prior efforts where only one type of loads, either mechanical or thermal, is considered, this paper included both of them in the modeling of strain transfer ratio and derived a general analytical expression for their relationships. It has been found that the strain transfer ratio is not a constant in some cases but varies with the strain being measured. The work studied the characteristics of fiber optic sensor in two consecutive approaches: 1) A simplified 2-dimentional multi-layer analytical model was built to derive the expression of strain transfer ratio as a function of the structural and material properties; 2) a numerical model that considers the realistic 3-dimentional structure of the sensor installation scenario was established for validating the analytical model in different case studies. Simulation results have shown that the analytical model matches well with the behavior of strain transfer ratio estimated by the numerical model, with an error less than 1.35%. Based on the validated analytical model, the discussion was further extended to derive the lower limit of the bonding length of optical fiber sensors to satisfy the requirement of measurement accuracy.