The working principle of two kinds of demodulation system of dynamic strains with FBG is deduced.One is based on unbalanced Mach-Zehnder（M-Z）interferometer and phase track direct current homodyne technique（PTDC）.The other is based on unbalanced M-Z interferometer and phase generated carrier homodyne technique（PGC）.The vibration experiment system is set up and the two kinds of demodulation system are compared.The experimental results show that the systems have good frequency response within 10kHz and the former has larger measuring scale and better effect on restraining the noise than the latter.The material impacting system based on the demodulation system of dynamic strains with FBG is set up.The experimental parameter is given and the experimental results show that the the signal to noise ratio（SNR）is about 80dB and the minimum dynamic strains variation is 10-4.The research testifies that FBG has nice performance of detecting dynamic strains.