To improve the performances of aero-engine nozzle in long service life and high infrared stealth， a detailed numerical analysis of the complex flow field in the divergent section of the vector nozzle is carried out. The interference between the secondary coolant and the main flow is firstly studied. Further， the influences of the complex flow field on the film cooling performance， the aerodynamic characteristics， and the intensity of the backward infrared radiation are analyzed. The results show that the secondary flow induces shock wave/expansion waves when injected into the supersonic mainstream. The complex waves further develop and impinge on the nozzle wall. Under constant mainstream and secondary coolant flow inlet conditions， the impingement locations of the complex waves move upstream with the increase of the vector angle. The film cooling efficiencies at the wave impingement locations decrease as the boundary layer rises. Additional pressure loss is induced when the mainstream flows through the complex waves， resulting in a decrease of the thrust coefficient and an increase of the total pressure loss， compared to the case without film cooling. The influence of the film injection position is considered. Compared to the downstream secondary flow injection， the upstream secondary flow injection leads to a lower film cooling efficiency， but a larger film coverage area. The effect of the number of coolant injection slots is also studied. With three slots， the average cooling efficiency on the upper and lower walls is 0.907 5 and 0.912 1， respectively， which increases by up to 92.55% and 98.37%， respectively， when compared to the condition with single slot. In contrast， the thrust coefficient and the total pressure recovery coefficient decrease by up to 1.62% and 2.20%， respectively. Then， the cooling and thrust performances of the gas mainstream are compared with the air mainstream. Taking gas as the working fluid， the cooling efficiency along the fully developed section of the secondary flow is lower， but the nozzle thrust coefficient is higher. Finally， the nozzle backward infrared radiation， the brightness of which corresponds to the nozzle flow field is also studied. In the wave bands 1–3 μm， 3–5 μm， and 8–14 μm， the injection of the secondary flow decreases the average radiation intensity by 67.39%， 56.58%， and 56.68% in the pitch plane， and 26.77%， 23.88%， and 23.24% in the yaw plane. The results of the study provide theoretical basis and basic database for the cooling designs of the vector nozzles.