A novel compact polarization beam splitter (PBS) based on a silicon nitride-silica-silicon horizontal slot waveguide is numerically proposed, to the best of our knowledge. In the slot waveguide, the transverse electric (TE) mode propagates through the silicon layer, while the transverse magnetic (TM) mode is confined in the slot region. By utilizing an asymmetric directional coupler (DC) and bent directional coupler (bent DC) consisting of a slot waveguide and a thin silicon waveguide, PBSs with 14.6 μm (DC) and 5.2 μm (bent DC) coupling regions are obtained. Without cascading other structures, the DC PBS shows a low insertion loss (IL) of 0.4 (0.2) dB and high extinction ratio (ER) of 31.56 (24.61) dB for a TE (TM) port around 1.55 μm, while the IL and ER of the bent DC PBS are 0.4 (0.15) dB and 35.83 (21.74) dB, respectively. The proposed PBSs also have a good fabrication tolerance and broad bandwidth (>40 nm) for an ER of >20 dB.