A Si-based gold (Au) wire-grid polarizer was designed for the 3 μm~5 μm mid-wave infrared (MWIR) wavelength range. The introduction of an MgF₂ dielectric layer between the Au grid and the Si substrate was proposed to improve the transmittance of transverse magnetic (TM) waves and the extinction ratio (ER) of the wire-grid polarizer. The structural parameters of the Au grid on the Si substrate were optimized using COMSOL simulation software. The results show that the average transmittance of TM waves is 93%, and the average polarized ER is 39.3 dB. In addition, a method of using tungsten (W) as a new metal material to improve the polarization performance of the wire-grid polarizer was proposed. The simulation results show that the average transmittance of TM waves is 92.7%, and the average polarized ER is 64.4 dB. However, the W has higher hardness and higher requirements on the etching process. It is expected to make superior MWIR wire-grid polarizers through advances in process and technologies.