The obtainment method of fiber Bragg grating (FBG) reflection spectra is analyzed, and according to the characteristics of reflection spectra, a theoretical method is proposed for improving the optimization objective functions in the demodulation algorithm of FBG strain distributions using correlation coefficients. The performances of improved and traditional algorithms are compared by simulation combined with the differential evolution algorithm. The simulation results show that the traditional algorithm is only suitable for the situation where the true reflectivity of FBG is known, in contrast the improved algorithm can be applied to the situation where the true reflectivity of FBG is unknown. The proposed method can be used to improve the practicality of demodulation algorithms of FBG strain distribution.