A new method to measure the beatlength of polarization maintaining fiber is shown. It utilizes the broadband light source and conoscopic interference phenomenon. The test system is composed of a broadband light source that is usually used in sensors such as the fiber optic gyroscope, two linear polarizers and the measured polarization maintaining fiber. After passing through the test system ,the shape of the broadband light spectrum will be modulated by the beatlength of fiber and becomes fluctuant. And as a result,the beatlength can be calculated by the modulation period. The measurement system is easy to be built up,and there is no strict demand on the operation precision and the polarizer performance. Any kinds of polarization maintaining fiber can be tested by the system. The measurement precision of the beatlength can be higher than 0. 1 mm.