By using of Kubelka-Munk theory,Mie theory and independent scattering approximation,the radiative transfer equation (RTE) was resolved in a infrared coating attached to a highly emissive substrate,and established the computation model of the emittance of the coating in the thermal infrared band(8～14μm). Taking aluminum particles with spherical shape as the pigments,according to the computation results,it' s given the dependence of coating emittance with respect to particle radius,thickness of coating. We proposed the optimum particle radius range of the pigment particles as well.