Based on CFD/IR numerical simulations, the effects of aspect ratio as well as relative space out of mixing duct on helicopter aerodynamics and infrared radiation characteristics were studied systematically. Conclusions can be drawn as follows according to the present simulation conditions: In comparison with IR suppressor with oval outlet, suppressor with rectangular outlet owns a stronger ejecting capacity, and its 3-5 μm infrared radiation intensity is relatively lower, which is exemplified by the fact that the maximum infrared radiation intensity decreased by 10.7％ by changing the shape of mixing duct outlet from oval to rectangle. With the aspect ratio of mixing duct exit rising from 2.2 to 4.6, the pumping ratios of the IR suppressors come to increase, but the total pressure recovery coefficients of those suppressors only change slightly. It can be the instrument of enhancing the pumping capacity when the relative space out of mixing duct is extended, which hardly contributes to greater pressure loss on the other hand. Whether the aspect ratio of the mixing duct exit is increased or relative space out of mixing duct is extended, the infrared radiation intensity(3-5 μm) for IR suppressors is going to decrease gradually in both horizontal and vertical planes.