Based on general CFD/IR numerical simulations, the effects of two-dimensional curved mixing duct geometric parameters on helicopter aerodynamics and infrared radiation characteristics were investigated. Conclusions can be drawn as follows according to the computational results: Compared with two-dimensional curved mixing duct with rectangular outlet, the distorted configurations of mixing duct lobed rectangular outlet extend the perimeter of exhaust exit, thus the strength of the secondary flow shed by lobes is enhanced due to the effective shear-mixing caused by viscous and shear force, which contributes to a maximum increase about 19.2％ in pumping capacity of the overall IR suppressor. Simultaneously, wall temperature of mixing duct is reduced significantly and the maximum infrared radiation intensity is decreased about 13.3％ to its highest when changing the shape of mixing duct outlet from rectangle to rectangular lobe. When the lobe heights of mixing ducts remain unchanged, with the increase of lobe numbers, the pumping ratio of IR suppressor firstly increases then decreases, while the total pressure recovery coefficient firstly decreases then increases. In addition, when the lobe numbers of remain unchanged, with the increase of lobe heights, the pumping ratio rises gradually, but the total pressure recovery coefficient keeps declining, and in this situation, the infrared radiation intensity peak achieve a maximum decrease about 11.6％.