Based on modes interference, twin-core optical fiber polarization beam splitter can spatially separate different polarized incident lights according to the coupling length difference of the two orthogonal polarization states. Making the dual-core optical fiber as a directional coupler, the power and polarization selectivity of the twin-core optical fiber polarization beam splitter are affected by the shape and size of the input and output waveguides when the light goes through the twin-core coupling area. Based on an elliptical twin-core optical fiber beam splitter and the RSOFT-Beamprop numerical calculation, the effects of the shape and size on the polarization selectivity, the bandwidth and fabrication tolerance are researched under the required extinction ratio (≥20 dB). The results show that the shape of twin-core optical fiber beam splitter is S-Bend, the input and output waveguide size is 40 μm and 4000 μm in the transversal X and longitudinal Z direction, respectively. Bandwidth of input light can greater than 7.5 nm and its coupled length is as comparatively short as about 209.87 mm.