Polarization measurement is able to effectively solve the problems that are beyond the reach of conventional photometry. When it comes to long-wavelength infrared (LWIR) polarization imaging system, polarization device plays a vital role in measuring the targets’ radiation and reflection and distinguishing them from busy background, which compensates for the lack of traditional thermal imaging if the difference in temperature is unobservable. Sub-wavelength wire-grid polarizer (WGP) is characterized by small volume and compact structure with the mi-cro- or nano-manufacturing technology. It is a grating structure whose period is smaller than the incident wave-length and when that is smaller than the critical one, the grating will only have zero-ordered diffraction, which helps improve the utilization ratio of polarization information. A dual-layered sub-wavelength grating consisting of two kinds of materials, aluminum and ZnSe, is developed to improve the performance of polarimetric elements in LWIR polarization imaging system. Parameters of the designed grating’s morphological structure are optimized on the basis of analyzing the effects on the polarization performances through the rigorous coupled wave theory, which helps describe the diffraction of electromagnetic waves by periodic grating structures and calculate diffrac-tive efficiencies of different orders. With a rectangular profile, the grating designed for applications in LWIR band has a structure of 1μm-period and 50%-fill-factor. The depths of aluminum and ZnSe in the grating region are 0.6 μm and 0.4 μm respectively. A TM transmission greater than 87.54% with an extinction ratio exceeding 47 dB is achieved in the 7 μm ~15 μm band when the angle of incidence is from zero to sixty degree. The grating maintains an extinction ratio better than 50 dB and TM transmission over 90.80% above 10.6 μm incident wavelength, which is superior to single-layered aluminum gratings with the same depth in the transmission performance in compar-ison. The structure is featured for the excessive etching on substrate, resulting in a series of air grooves. Therefore, the dielectric grating layer beneath the metal wire grid is formed. This method for improving polarization perfor-mances is easier to implement than coating anti-reflective films. It is investigated that the TM transmission in-creases with the depths of both metal and dielectric layers when the extinction ratio is dominated by the depth of metal layer, while the single-layered ZnSe grating shows little potential in extinction ability for the lack of metal component. Compared with the existed designs of WGP, the simulation results show that the TM transmission and extinction ratio are effectively improved in broad LWIR band with the proposed structure. Besides, the an-gle-tolerance indicates that the design has great capability in applications with wide field angle.