When a hypersonic vehicle maneuvers at a high angle of attack, the jet generated by its off-orbit engine interferes strongly with the high-speed thin atmospheric flow, and the flow field is complicated, and the infrared radiation generated by the flow field is also a landmark event in space-based infrared system detection. In this paper, aiming at interference situation of the jet flow of hypersonic flight vehicle engine and thin flow, Navier-Stokes equations are numerically solved to simulate the interference flow field, and the infrared radiation characteristics of gas are obtained by the line-by-line integration method. Combining with the backward Monte Carlo method, the infrared radiation characteristic of exhaust plume are obtained when the aircraft flight′s altitude is 94 kilometers, with no wind, different incoming flow attack angles and different velocities are considered, and the observability of low orbit satellites is evaluated. The simulation results show that for a given observation position, the intensity of infrared radiation in each band is low when there is no wind, and the maximum value is 10^-9 W/m². Under the influence of incoming flow attack angles, the infrared signal intensity of the flow field increases significantly with greater attack angle and velocities and the maximum value reaches 10^-6 W/m². The atmospheric attenuation effect has great influence on the observability of different observation positions. The results can provide reference for infrared warning and anti-missile of hypersonic vehicle.