The research of infrared radiation characteristics of high temperature free-stream flow with particles is very important for the field of target detection, combustion diagnosis and temperature measurement of flame. In this study the infrared radiation characteristics of high temperature free-stream flow were calculated with backward Monte-Carlo method. The effect of the particle scattering and different boundary conditions on the directional radiation heat flux was examined. The calculation results were compared with forward Monte-Carlo method. The result shows the superiority in computational efficiency of backward Monte-Carlo method.