Beat length is an important parameter to characterize the performance of polarization maintaining fiber, with the widely application of polarization maintaining fiber in the field of fiber communication and sensing, it is very important for the high accuracy measurement of beat length. Based on the white light interferometry, a beat length measuring device of polarization maintaining fiber is developed. Beat length is calculated by measuring the geometric length between the two stress points on the polarization maintaining fiber and the optical length between the corresponding coupling points. Geometric length is accurately measured by laser interferometer, and the measurement of optical length is firstly by using Hilbert transformation and polynomial fitting to extract white light interference envelope signal, obtain the precise peak position of the polarization coupling point, and then, the frequency stabilization interferometry is used to measure the optical length between the coupling points. The experimental measurement results and uncertainty analysis indicate that the measurement repeatability is better than 4 μm, and the measurement uncertainty is 20 μm.