In nanoimprint lithography of infrared metallic gratings, the grating structure is easily damaged by hard template nanoimprint lithography and their polarization characteristics would be reduced. This paper proposes a flexible nanoimprint lithography as the alternative method to fabricate the bilayer metallic nano grating with a height of 100 nm and a thickness of 40 nm. The grating is suitable for working at 3-5 μm and its main parameters are the period in 200 nm, line width in 100 nm, and the aspect ratio in 1∶1. In fabrication, the master template grating structure was copied to the Intermediate PloymerSheet (IPS) material by thermal nanoimprint lithography to obtain IPS soft template for the embossing. Then, the IPS grating structure was transferred to the STU-7 resist by UV-embossing lithography to get the dielectric grating with complete and uniform structure. Finally, Al was deposited on this grating by vertical thermal evaporation, and the mid-infrared bilayer metallic nano grating was successfully fabricated. The fabricated grating was tested. The results show that the transverse magnetic transmittance of this mid-infrared bilayer metallic nano grating is greater than 70% in the 2.5-5 μm, and its extinction ratio is more than 30 dB in the 2.7-5 μm, especially it is greater than 35 dB in the 2.72-3.93 μm, showing excellent extinction ratio and polarization characteristics. These results demonstrate that the grating fabricated here has potential applications in infrared polarization detection and infrared polarization sensing.