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A short polarization beam splitter (PBS) is presented based on an asymmetrical evanescent coupling system, which consists of a narrow input waveguide, a narrow output waveguide, and a wide middle optical waveguide between them. The width of the waveguides is designed so that the phase-matching condition is satisfied for the TM fundamental (TM0) mode in the narrow input/output waveguide and the first higher order TM (TM1) mode in the wide middle waveguide. Meanwhile, there is a significant phase mismatch for the case with TE polarization. Therefore, for the launched TE polarized light, almost no coupling happens when it goes through the coupling region and finally the TE polarized light is output from the through port. For the launched TM0 mode in the narrow input waveguide, it is completely coupled to the TM1 mode in the wide middle waveguide by choosing the optimal length of the coupling region. Furthermore, the TM1 mode excited in the wide middle waveguide is then coupled to the TM0 mode in the narrow output waveguide through the evanescent coupling between them. A short (~25 μm long) PBS is designed based on silicon-on-insulator nanowires, while the gap width is chosen as large as 300 nm to make the fabrication easy. Numerical simulations show that the present PBS has a good fabrication tolerance for the variation of the waveguide width (more than ± 20 nm) and a broadband ( ~ 50 nm) for an extinction ratio of >; 15 dB.