Controllable polarization beam splitter is a new type of optical waveguide functional device, which plays an important role in improving the performance of integrated photon system or developing new applications. A novel controllable polarization beam splitter is proposed by utilizing the large birefringence effect and high tunability of liquid crystal, in which the multimode interference optical waveguide structure is adopted. The optical performance of the polarization beam splitter is simulated and analyzed by the beam propagation method. The results show that, the polarization beam splitter is easy to achieve a high extinction ratio and a low optical loss. The polarization extinction ratio is over 28.7 dB and optical loss is less than 0.024 dB for both transverse electric (TE) wave and transverse magnetic (TM) wave polarizations. And the output ports of TE wave and TM wave can be dynamically controlled by adjusting the orientation of the crystal axis of liquid crystal. This device has several advantages such as simple structure, ease for design and manufacture, ease for integration with other photonic device, which has a wide potential application prospect in the photonic integrated circuits.