Coupling coefficient will directly influence the light distribution of linearly polarized light through the Fused-Tapered zone of Polarization Maintaining Fiber(PMF) couplers. Accordingly, it affects the coupling performance of the couplers. Coupling model of fused polarization-maintaining fiber couplers is built on the basis of coupling mode theory of optical waveguide. A new formula for coupling coefficient of PMF couplers has been derived. It is appropriate for any PMF when the core is circular and the birefringent axes are nonaligned. The conclusion of the research can provide the precondition to solve the coupled mode equation and analyze the performance of couplers. Therefore, theoretical direction for the high-performance manufacturing of PMF couplers is provided.