To achieve accurate strain measurements using fibre Bragg gratings (FBGs), both the sensing and wavelength demodulation accuracy are worth considering. Due to the indispensability, influences of bonding materials on the measurement accuracy need to be clarified. To analyze the sensing accuracy of the FBG strain sensor, a simplified longitudinal strain transmission model is established to discuss the transfer coefficient issue. Results indicate that hard bonding materials can enhance the sensing accuracy by high strain sensitivity. But on the other hand, asymmetric transversal stress model reveals that hard bonding materials can lead to the distortion of the spectrum in a large load condition. Then, effects of distorted spectra on the wavelength demodulation accuracy are studied by the simulation. The simulation shows that distorted spectra can reduce the wavelength demodulation accuracy markedly. It can be concluded that to improve the accuracy of strain measurements, hard bonding materials are suitable for small strain measurements, and relatively soft bonding materials are better options for large strain measurements.