The thermal radiation field in a high temperature gas flow is coupled with the flow field. However, within the temperature range of the exhaust flow field for a jet engine, the thermal radiation effects can usually be ignored in the computations of the flow field, and the governing equations on the flow field and the radiation field can be decoupled. A decoupling model for computing infrared signatures of the flow fields surrounding the aircraft was proposed. A numerical procedure was constructed by first utilizing TVD(total variation diminishing) scheme to solve the interactive flow field. A finite volume formulation for the spectral intensity was imposed to estimate the total infrared signaturesusing the computed flow field data over the same set of grids. As a test case for validating the proposed numerical procedure, the infrared signatures in an interaction flow field of a uniformed external flow and a frozen exhaust flow with 7 species, N2, O2, CO2, H2O, CO, HCl, and H2, were computed and compared favorably with the available experimental as well as numerical results.