The performance of a terahertz (THz) Brewster polarizing beam splitter on polymer substrate is studied theoretically and experimentally. Simulations by using finite-element method demonstrate that both transmittance/reflectance and their extinction ratios are better than its 45 counterpart in a broad frequency range. Especially, the reflection extinction ratio improves significantly. The results are also verified experimentally with THz time domain spectroscopy (TDS) and backward wave oscillator (BWO) measurement system.