To achieve an accurate measure of the strain value of the measured structures， a metallic packaged fiber Bragg grating （FBG） strain sensor is designed. A six-layer strain transfer model of the FBG sensor is established， and its strain transfer mechanism from the measured substrate layer， substrate adhesive layer， packaging substrate layer， adhesive layer of FBG， protective layer， and FBG layer is analyzed and derived. Through theoretical analysis and finite element simulation， the effects of the FBG length， the thickness of the FBG layer and substrate bonded layer， the elastic modulus， and Poisson's ratio of middle layers on the average strain transfer rate are comparatively analyzed. According to the simulation results， the packaging material， size， and adhesive layer thickness of the designed FBG sensor are determined. Finally， an equal-intensity cantilever beam that provides the load on FBG is designed， and a correction coefficient of the strain transfer error is proposed. A comparison test of strain transfer is carried out on the bare FBG and metallic packaged FBG sensors. The results show that their average strain transfer rates are 98.25% and 98.15%， respectively， and the error between the simulation results is within 0.15%~0.25%.