The method of the characteristic matrix was used to analyze the suppression effect of photonic crystals on atmospheric window band emission. Several combinations of substrates and photonic crystals were considered, also discussed were photonic crystals construction and spontaneous radiation. It is found that, when the substrate is aluminum at 100°C, with photonic crystals consisting of Si (of high refractive index) and KCL (of low refractive index), the emissive power between 3∼5 μm of the combination surface is reduced to 0.101 W∕m², for comparison, the corresponding emissive power of aluminum surface is 0.302 W∕m². With photonic crystals, the emissive power between 8 ∼ 14μm of aluminum surface is reduced from 2.050 to 1.304 W∕m². Substituting Ge for Si, the emissive power of the combination surface is 0.044 W∕m² between 3 and 5μm and 0.219 W∕m² between 8 and 14μm . When the substrate is an imaginary surface (n(ω)=1, κ(ω)=0) at 100°C, with photonic crystals consisted of Si and KCL, the emissive power of the combination surface is 2.779 W∕m² between 3 ∼ 5μm , for comparison, the emissive power of the “imaginary surface” is 15.765 W∕m² at the same temperature. Using photonic crystals, the emissive power between 8 and 14¼m of the imaginary surface could be reduced from 138.658 to 30.621 W∕m².