The dispersion measurement of optical fiber is realized by utilizing the relationship between the multi-longitudinal mode beat frequency of the laser and dispersion coefficient of resonant cavity. A narrow band and wavelength tunable reflector is used as the rear-cavity mirror of the fiber laser in the proposed system, which is based on the combination of the fiber Fabery-Perot tunable filter and the optical fiber circulator. The Sagnac interferometer is used as the front-cavity mirror. The tested optical fiber is inserted in the resonator. The beat frequency of the fiber laser is measured by the spectrum analyzer, which is changed with the output wavelength of laser, and then the dispersion coefficient of the sample is obtained. The dispersion coefficient of dispersion compensation fiber and standard single mode fiber are measured respectively. The experimental results indicate that the measurement accuracy of the proposed system can meet the requirements of the communication optical fiber measurement. The proposed method has the advantages of high measurement speed, low cost, simple structure and good application prospect.