Using the method of optical pulse delaying, a novel fiber length measurement method is proposed. The method is that the modulated light source is divided into two parts by the optical fiber coupler and goes through the measured optical fiber and the reference optical fiber. The frequency of modulation signal is regulated so that the two signals can coincide, and then the measured optical fiber length is calculated by analyzing the modulation frequency. Compared with the traditional optical time domain reflectometer, the measurement precision is improved from the meter scale to the centimeter scale. Compared with the optical frequency domain reflectometer, optical coherence domain reflectometer and frequency-shifted asymmetrical Sagnac interferometer, the requirement of stability and coherence properties of light source is lower and the system is more easier to be achieved. The experimental result indicates that fiber length measurement method based on optical pulse delaying has a large dynamic range and a high precision. The maximal multimode optical fiber lengths measured by this method at 850 nm and 1300 nm are 10 km and 20 km, respectively, and the measurement accuracy can reach centimeter magnitude.