The invention relates to an optical fiber length measuring method and device and belongs to the technical field of optical measuring. The method includes: modulating microwave signal strength in a continuous optical carrier to generate a modulated optical signal; using the modulated optical signal as the probe light, injecting the modulated optical signal from one end of a to-be-measured optical fiber, and receiving the reflecting optical signal of the second end of the to-be-measured optical fiber from the first end of the to-be-measured optical fiber; converting the reflecting optical signalinto an electric signal, extracting a base-frequency signal from the electric signal, and measuring the phase change of base-frequency signal relative to the microwave signal; acquiring the length ofoptical fiber according to the phase change. Compared with the prior art, the optical fiber length measuring method and device can perform high-precision measuring on large-range optical fiber length.