Based on phase modulation optical link, a novel fiber length measurement system is proposed. The theory model of gain of optical link is derived. By analyzing the relationship between the free spectral range of gain of the optical link and the optical path difference of the fiber interferometer, the fiber length is acquired. Compared with the traditional time domain reflectometry (OTDR), this system has no dead zone, and improves the precision from meter to millimeter. Compared with optical frequency domain reflectometry (OFDR) and optical coherence domain reflectometry (OCDR), this system does not need costly and high-performance laser to ensure coherence. The feasibility of the system is analyzed in theory, and is verified by experiments. It is shown that the smallest measurable relative length of fiber is 3 mm with the existing 67 GHz vector network analyzer, and the error can be controlled within 0.05 mm when the relative length of fiber is 100 m. This system is characterized by simple structure, easy operation and higher practical value.