Abstract: A new method to measure the beat-length in low-birefringence fibers was shown. The optical path was consisted 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. Phase detection was chosen to reduce the error from environment influence. Phase compensation plate was applied to ensure the testing system always sensitive to the change of phase. The testing system was set up and tested in reality. The system performed 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 measured fiber. It is reliable for fibers witch's beat-length is up to 20 m. The beat-length precision of the measurement is 1%.