Polarizing beam splitter occupies a very important position in the generation and control of vector beam, the demand for it has been increasing. Metasurfaces provide a highly flexible platform for regulating the propagation of electromagnetic waves. In this paper, a planar polarization beam splitter based on dielectric metasurface at wavelength of 118.8 μm, is demonstrated and simulated. It is mainly achieved by the array composed of silicon cubes of different sizes placed periodically on a silicon dioxide substrate. The three-dimensional finite-difference time-domain method was used to numerically simulate the optical characteristics of the polarizing beam splitter. The polarizing beam splitter can transmit vertically incident polarized light into x polarization and y polarization propagating in different directions, and the efficiency is as high as 90%. As an example, to further demonstrate the performance of the polarization beam splitter, we also design a focusing lens based on the hyperboloidal profile theory at λ=118.8 μm. The lens can focus differently polarized light beams into one spot in different directions. With appropriate design, we expect this structure can be applied to many practical applications with high efficiency in transmitting mode in terahertz range, such as vector beam generators, polarizers and so on.