CO₂ is the main source of radiation noise in mid-infrared and short-infrared detection. The research on radiation and transmission property of CO₂ at high temperature is still not clear at present. The absorption effect of CO₂ in the shock layer was mainly studied, and the thermal radiation of CO₂ and radiation component proportion on the photosensitive surface were analyzed and calculated. The improved tangent-slab approximation method was used to solve the radiative transport equation, and the influences of target and window radiation on the radiation field distribution in the shock layer were considered. Based on this, the radiation and transmission property of shock layer were deduced. Finally, the calculation model of each radiation component and proportion of the photosensitive surface was given. The property of the shock layer of a supersonic missile at an altitude of h=1 km and flight speed of Ma=3-5 was simulated and calculated. The results showed that the transmittance of shock layer in mid-wave bands is lower than that in short-wave band, but shock layer absorption can be ignored in general. The increase of flight speed leads to a broadening of the CO₂ radiated noise region to the long-wave direction, and the target signal is submerged seriously by CO₂ radiation noise in the band above 4.4 μm under condition of Ma≥4 and h=1 km, the filter with a cutoff wavelength of 4.4 μm is not suitable.