For anti-resonant fibers (ARF), when the diameter of cladding tube approaches the effective diameter of fiber core, the fundamental mode (FM) in core appears an index-induced mode coupling with the FM in cladding tubes. The FMs mode coupling between core and cladding forms a super FM in ARF, and simultaneously introduces an obvious suppression on the FM in core, which provides a novel theory for polarization-maintaining ARF. In this paper, we proposed a polarization-maintaining ARF (PM-ARF) with four asymmetrical cladding tubes for 2.5 THz wave. In PM-ARF, the horizontal cladding tubes are designed to generate the suppression on modes, and the vertical tubes are designed to generate the guidance for modes. Profiting from the mode coupling between core and cladding, a birefringence index of 9×10-6 is obtained by the compact PM-ARF. The mode coupling was confirmed can be combined with methods of bi-thickening the core boundary and elliptical core boundary respectively to improve the polarization-maintaining ability. Optimizing the shells of horizontal and vertical cladding tubes into different thickness respectively, and simultaneously working with the mode coupling between core and cladding, the birefringence index of ARF was improved to larger than 1×10-4. Besides, the mode coupling worked with the method of elliptical core boundary, the birefringence index was further improved to higher than 2×10-4.