The measurement accuracy requirement for fiber length is higher and higher in the new generation fiber sensing system. A fiber length measurement method based on Sagnac interferometer is proposed. The basic principle and the experimental setup of the proposed method are introduced. Experiments of fiber length measurement are conducted. The influences of polarization state of light source and fiber disturbance on measurement results are verified. Results show that the measurement accuracy of the proposed method can reach centimeter scale and the measurement range is from 1 km to 10 km.