The Direct Simulation Monte Carlo method was used to describe the motion, collision and energy transfer effects of a finite number of simulated molecules. The non-equilibrium flow field characteristics of the high-altitude plume were calculated, and the macroscopic parameters of the plume were obtained by statistical average method. On this basis, the Voigt line function was used to describe the broadening of the thin gas, and line-by-line integration was used to obtain the narrow-spectrum gas radiation. The physical transform equation, combined with the backward Monte Carlo method, were used to calculate the radiation transfer equations of the high-altitude plume. The applicability of the flow and radiation calculation models was verified using theoretical and experimental data. The flow and narrow spectrum infrared radiation characteristics of a small thrust engine were calculated and analyzed by using the above model. The results show that: due to the rapid expansion of the high-altitude plume, the density decreases rapidly, which leads to a significant non-equilibrium effect; the gas is affected by the velocity inertia, and the gas of different molecular weights will have a diffusion separation effect; the gas spectrum is thin and narrow, with Doppler broadening, and the peak of radiation moves to the middle wave, 4.7 μm CO and 6.5 μm H2O emission band radiation energy account for a larger share. The radiance in the axis direction is concentrated within the distance of twice the diameter of the nozzle, while the radiance distributed in the radial direction is concentrated in the shock region and the inner flow region within the separation wave line, and the radiance in other regions decreases exponentially.