The invention belongs to the technical field of spectrum analysis and discloses a spectrum analytical device and a spectrum analytical method based on Brillouin dynamic grating. The spectrum analytical device based on the Brillouin dynamic grating comprises a laser, an optical fiber coupler, a first fiber optic amplifier, a first isolator, a first polarization controller, a second polarization controller, a single side band modulator, a second fiber optic amplifier, a second isolator, a third polarization controller, single mode fibers, a polarization beam splitter, a circulator, a photoelectric detector, a data collection card, a fourth polarization controller and a microwave source. According to the spectrum analytical method based on the Brillouin dynamic grating, the Brillouin dynamic grating in the single mode fibers is utilized to be served as a light splitting element, a super narrow grating reflection bandwidth is provided due to the fact that the Brillouin dynamic grating can be processed to be very long, so that spectrum analysis can be achieved according to a finally obtained complete spectrum image of an optical path to be measured. The spectrum analytical device and the spectrum analytical method based on the Brillouin dynamic grating solve the problem that the existing light splitting element of the spectrometer cannot guarantee that high resolution cannot be obtained under the condition of a large measurement range and are applied to the spectrum analysis.