A sub-wavelength periodic grating has special characteristics that a traditional grating does not have. We, therefore, fabricated a sub-wavelength, metal, nanograting polarizer on a square of polycarbonate (PC) using nano-imprinting technology. The grating period is 278 nm, depth is 110 nm, duty cycle is 0.5, and deposited aluminum-metal layer is 70 nm thick. The performance of the sub-wavelength metal grating polarizer was tested using a spectroscopic test system. The experimental results show that when the incident wavelength is 600 nm, a double-layer flexible grating polarizer acquires good polarization characteristics. Its transmission efficiency for TM-polarized light is as high as 55%, and the extinction ratio is as high as 32 dB. The performance of the polarizer was tested using a six-channel sensor made in the laboratory. The test results show that the average error of the polarizer is 0.2002°, maximum error is 1.105°, and standard error is 0.7255°. The manufacturing process only involves two steps: the nano-imprinting process and the metal-evaporation process; it does not involve coating, stripping, or etching an imprint adhesive. So the manufactured gratings have obvious advantages for low-cost mass production of large-area polarizers, which can be widely used in the production of semiconductor optoelectronic devices such as optical-detection devices and optoelectronic switching devices.