A compact and broadband polarization beam splitter (PBS) is proposed based on an asymmetric directional coupler consisting of a hybrid plasmonic waveguide and a ridge waveguide on lithium-niobate-on-insulator. Due to the surface plasma polariton (SPP) effect, the phase-matching condition is satisfied for transverse electric polarization, whereas the transverse magnetic polarization has a significant phase mismatch by choosing reasonable widths of both waveguides. A short ( ∼ 28 μm long) PBS is designed while the gap width is chosen to be 200 nm to make it easy to fabricate. Numerical simulations show that the designed PBS has a broad bandwidth (>130 nm) for an extinction ratio of >15 dB and a large fabrication tolerance for the variation of the waveguide width (over ± 50 nm).