The invention relates to a high-sensitivity rapid demodulation system for a small change of a comb-like spectrum. The high-sensitivity rapid demodulation system comprises a three-port circulator; a chirp grating is arranged the port II of the three-port circulator; a long-period grating and a phase modulation type fiber sensor connected with the long-period grating are arranged at the port III of the three-port circulator; and a photoelectric detector capable of receiving optical power conversion is arranged next to the long-period grating and is connected with a signal processing, collection, and display system. Therefore, a problem that a conflict between a measuring range and a measuring speed exists and the measuring range and the measuring speed can not be improved simultaneously according to the exiting comb-like spectrum demodulation technology can be solved.