Abstract: CVD diamond is an excellent material for infrared optical window, but its theoretical transmittance in the infrared band can only achieve about 71%. The optical transmission performance of the CVD diamond film can be improved by the surface sub-wavelength structure design. In this study, the quantitative relationship between diamond microstructure characteristics and optical antireflection was established through theoretical simulation. According to the theory guidance, the CVD diamond film with surface microstructure was fabricated by replicating the Si substrate through MPCVD method to improve the transmittance of diamond in the infrared band. Scanning electron microscope (SEM) was used to observe the surface and microstructure of the original silicon wafer and diamond. The growth layer quality and the nucleation layer quality of diamond were both evaluated by Raman scattering spectrum. Infrared spectrometer was used to test the infrared transmittance of diamond film. The results show that after constructing the microstructure on one side, the transmittance of the diamond film in the 8-12 μm band can be increased from 70% to 76%, which means the surface microstructure can significantly improve the optical transmission performance of diamond film. The non-diamond nucleation layer and the insufficient integrity of surface microstructure may be the main reason for the gap between the experimental results and the theoretical simulation results.