Abstract: Several double-S nozzles with different outlet shapes were designed based on an axis-symmetric nozzle to analyze influence of the outlet shape on the infrared radiation(IR) characteristics of exhaust system. The curvature regulation method and mass-flow matching principle were adopted in the design. The distribution of the characteristics of flow field and mass fraction of six exhaust system models were obtained by CFD. The IR characteristics of these models were calculated by reverse Monte-Carlo method (RMCM), and then IR intensity space distribution of the rear hemisphere, radiation brightness distribution and the lock distance of the rear hemisphere were researched. The results show that, the double S-shaped nozzle can reduce the infrared signal of target effectively. The outlet shape can affect the IR characteristics of exhaust system. The models with trapezoid outlet shape has significantly lower IR intensity than the double S-shaped nozzles with other outlet shapes, and can reduce the peak of IR intensity at least 70% compared with the axis-symmetric nozzle. Reducing trapezoid's bottom angle can enhance the infrared suppression effect, and appropriate outlet shape can further suppress the IR intensity of exhaust system within acceptable thrust loss.