A subwavelength polarizer based on "sandwich" structured substrates is proposed in this study. The proposed subwavelength polarizer consists of three layers of subwavelength aluminum wires and dielectric substrate. The designed structure achieves an extinction ratio (ER) greater than 90 dB in a 400-800 nm visible wavelength region, achieving a maximum ER of 135 dB at 750 nm. Our results demonstrate significant improvements over the conventional single- and double-grid polarizers in terms of an ER and spectral range coverage. The proposed subwavelength polarizer in this paper has great potential in polarimetric imaging, liquid crystal display, and other optical fields.