Physical models of three rectangular nozzles with different aspect ratio were established, and the 3-D exterior flow fields of these rectangular nozzles were simulated through numerical method with the software Fluent6.3. Then the data of their flow fields such as temperature, pressure and density and so on were obtained. According to the characteristics of their plumes, the computational areas of infrared radiation were established. Through statistical narrow-band model based on Lorentzian profile, mean absorption coefficients of the plumes in narrow band l were gained. Then radiative transfer equations with gas medium were solved by Finite Volume Method (FVM). Finally, the spectral characteristics and the total intensity distributions in 3～5 μm of those exhaust plumes infrared radiant were acquired. The result improves that two peaks of plume infrared radiant merge in 2.7 μm and 4.3 μm and the infrared radiant of exhaust plume is weakened greatly with the increase of aspect ratio.