Based on the photo-elastic effect and index ellipsoid, the relationship between transverse force and dielectric constant variation of polarization maintaining fiber(PMF) was obtained under the consideration of the difference between isotropic and anisotropic optical fiber under transverse force. Polarization coupling characteristic of PMF under transversal force was analyzed based on coupled-mode theory.The relationships between the coupling intensity and the amplitude, direction and the acting length of the transversal force were clarified by numerical simulation. When the direction of force was 45°, the coupling intensity of PMF was the best sensitive to the transversal force.The output extinction ratio changed periodically with acting length of the force, and different amplitude of the force corresponded to different period. The linear relation between the output extinction ratio and the amplitude of transverse force in the part region was found. These characteristics are consistent with previous experimental results.