A measurementof beat-length in low-birefringence fibers based on wavelength modulation is shown. It measures the beat- length by wavelength modulation. Phase detection is chosen to reduce the error from environment influence. Phase compensation plate isused to ensure the testing system always sensitive to the change of phase.The optical path consistes of an amplified spontaneous emission (ASE) light source, a fiber Fabry-Perot tunable filter, two linear polarizes, the measured fiber and a phase compensation plate. The testing system is set up and tested in reality.The system performes well in the repeated test. It is simple and has no strict demand on these equipments. There is no limitation on the configuration and the length of the testing fiber in the measurement.It is reliable for fibers whose beat- length is up to 20 m. The beat- length precision of the measurement is less than 1%.