In order to solve the problem of fine measurement of FBG demodulation system in image sensor,a weak signal measurement method based on non apodized FBG is proposed.A non apodized FBG reflection spectrum approximation model was established,and the method of logarithmic amplification was used to improve the sidelobe of non apodized grating,the spectral was demodulated in a wide range of grating spectrum and the demodulation algorithm was given.Based on the Monte Carlo method,the non apodization grating was measured and simulated,and the experimental system was built to compare the measurement of the non apodization grating and the wide spectrum Gauss grating.The results show that the measurement method of weak signal based on non apodized FBG can effectively improve the accuracy of wavelength measurement.By correlation demodulation,the measurement accuracy can reach 2 times of wide spectrum grating.