An innovative polarization coupler based on a long-range surface plasmon-polariton waveguide, which demonstrates efficient coupling between a semiconductor laser diode and a polarization-maintaining fiber, is proposed and analyzed. The proposed coupler—with different coupling facet sizes—can simultaneously match the mode sizes of the semiconductor laser diode and the polarization-maintaining fiber and can achieve polarization light coupling between different devices according to the features of the long-range surface plasmon-polariton waveguide. The coupling efficiency and alignment tolerance of the proposed coupler are investigated using the numerical-analysis method, and the design is optimized to obtain the best properties of the coupler. The calculated optimal coupling efficiency and the insertion loss are 89.96% and 0.97 dB, respectively. Further, the device exhibits a good alignment tolerance and has widespread potential applicability in the field of optical interconnections.