The reflectivity spectrum of the fiber Bragg grating(FBG) will get wider when loaded in the unhomogeneous strain field, and error is brought into the measurement. Linear filtering method is used for high frequency wave demodulation. The demodulation process was simulated by transmission-matrix method with typical FBG parameters.The results show that the measurement error is lower than 3% when the peak strain is sticked to 3000 με and the strain wavelength is larger than 0.0025 m. The measurement error is lower than 0.25 % when the strain wavelength is 0.05 m and the peak strain is smaller than 10000 με. The numerical result agrees well with the experiment on the slide Hopkinson pressure bar.