A high extinction ratio (ER) wideband mid-infrared polarization beam splitter based on a GaAs hybrid plasma tridirectional coupler is proposed; it consists of a Si₃N₄–GaAs waveguide (SGW), GaAs waveguide, and GaAs hybrid plasma waveguide. TE0 and TM0 are coupled to the SGW and GaAs waveguide, respectively, by adjusting the width of the waveguide to meet the phase matching condition. When the wavelength is 3.5 μm, the ER and insertion loss (IL) of TE (TM) mode are 48.9 (51.85) and 1.36 (0.3) dB, respectively. In the 400 nm operating bandwidth range of 3300 to 3700 nm, the ER of TE and TM modes is >22 dB. The designed polarizing beam splitter has a high ER and low IL. Because of its large signal transmission capacity, it has potential applications in air molecule detection and analysis and optical communication.