We report on both the theoretical and experimental design of a black phosphorus (BP)-based reflective linear polarizer on Si/SiO2 substrate in visible range using the Fabry-Perot cavities method. Thanks to the optical anisotropy of BP, polarization wavelength regulation and a high extinction ratio are achievable via optimizing the thickness of BP. Using azimuth-dependent reflectance difference microscopy, we directly measured a huge optical anisotropy of 1.58, corresponding to an extinction ratio of ∼9 dB, from a 96 nm BP on a silicon substrate capped by 260 nm thermally oxidized silicon at a wavelength of 690 nm for the first time, to the best of our knowledge. Our results not only provide a new route to designing nanoscale polarizers based on anisotropic two-dimensional (2D) materials, promoting the application of 2D materials in integrated optoelectronics and system-on-chip, but also suggest a modulation technique for optical anisotropy by integrating the BP film with cavity structures.