An optical fiber sensor used in the simulating experiment of marine natural gas hydrates to measure the phase change of marine natural gas hydrates was introduced.The intensity change of transmission light during hydrates states change was measured by the sensor to determine the phase change.Dual optical path construction was employed in the optical fiber sensor in order that a high measuring precision would be obtained.The light intensity of reference beam was measured to compensate the fluctuation of light source while the intensity of transmission light was measured simultaneously.Besides,dual detectors were used in each photoelectric transfer circuit to eliminate the influence of dark current of the photodetector.