CVD diamond window which can be used in high power CO2 laser was prepared. Firstly, a 2 inch diameter diamond self-supporting film was deposited by a circumferential antenna ellipsoidal cavity type MPCVD reactor. Then both sides of the film were polished, and the film was cut into rectangular samples by laser. An antireflective film with the central wavelength of 10.6 μm was prepared on the surface of the sample by vacuum evaporation. Finally, the diamond optical window was prepared. In this study, the infrared transmittance before and after coating, thermal conductivity and burst strength of diamond substrate were respectively measured by Fourier transform infrared spectroscopy, thermal conductivity meter and pressure blasting test stand. Using a self-built optical platform, the laser power density that the CVD diamond substrate antireflection coating can withstand was tested. The results show that the transmittance of CVD diamond substrate at 10.6 μm is 70.9%, the absorption coefficient calculated by spectrum is 0.06 cm-1, the thermal conductivity is more than 19.5 W/(cm·K), the burst strength is more than 5.62 MPa, the transmittance with coating is 99.2%, and the laser power density that the AR coating can withstand>995 W/mm2.