In this paper, we have proposed a novel antireflection coating with multilayers of SiO2/Si3N4 on a silicon window, suitable for performance evaluation on a microbolometer thermal sensor in the infrared range of 8–12 µm. The 4-layer coating (2 periods of SiO2/Si3N4) with optimized thickness was designed using the Essential Macleod program developed by Thin Film Center, Inc. On the basis of the proposed long-wavelength infrared (LWIR) silicon-based window by simulation, the samples were also fabricated by physical vapor deposition to characterize and achieve the best possible agreement by Fourier transform infrared (FTIR) spectroscopy. It was found that, in the wavelength range of 8–12 µm, the average transmittance of the double-side-coated sample increases by about 39% compared with that of the uncoated silicon window and its maximum reaches about 98% at 9 µm. We have also demonstrated the performance of an uncooled microbolometer sensor using the coating multilayers of SiO2/Si3N4 on a silicon window, which is compatible with the commonly used germanium window.