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 long-wavelength infrared (LWIR) polarization imaging system. Parameters of the designed grating’s morphological structure are optimized on the basis of analyzing the effects on the polarization performance through the rigorous coupled wave theory, which helpfully calculates the diffraction efficiency. With a rectangular profile, the grating designed for applications in LWIR band has periods of 1 μm 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 7~15 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 comparison. The simulation results show that this grating has excellent polarization performance in the broad LWIR band. Key