A precise measurement of optical path difference of optical fiber interferometer is proposed which is based on the fiber Bragg grating demodulation system using Fabry-Perot filtering technology.The principle of Fabry-Perot filtering technology is introduced.By analyzing the principle of unbalanced Michelson′s interferometer, the relationship of interferential spectrum and arm length differences are indicated, and the principle of precise measurement of arm length differences by fiber Bragg grating demodulation system are analyzed.The experiment demonstrates that the arm length differences can be measured with a accuracy of 2μm.