The mechanic model of surface fiber Bragg grating (FBG) sensor is built to study the strain transfer between the composite beam and the surface FBG sensor. The relationship between the actual strain of composite beam and the strain of surface FBG sensor is deduced. Various factors which influence the average strain transfer rate of surface FBG sensor have been analyzed. Modulus elasticity, width of the cementing layers length of the fiber pate are primary factors to influence the strain transfer rate. A optimum combination of the three factors is determined through an experiment, which proides reference for practical engineering application in future.