Abstract: In practical application, the light injected to polarization maintaining fiber（PMF）-is typically quasi-monochromatic. When the polarization direction of the incident light and the birefringence axes of the fiber were misaligned, the degree of polarization(DOP) would be changed in transmission process. Using partial coherence theory, the dependence of polarized and partially polarized lights DOP on the starting angle and transmission distance in PMF were analyzed theoretically. And some experiments were done to study the dependence of partially polarized light's DOP on the starting angle. The results show that, if the transmission distance is in centimeter magnitude, the influence on DOP is great. To normal long distance transmission, its influence could be neglected. Compared with the transmission distance, DOP varies greatly with the starting angle. To achieve the relative change of DOP less than 0.01%, the starting angle must be smaller than 0.405.