Mechanism analysis of aero-optical transmission effects on infrared imaging at 2.5Ma is carried out. Parameters of external flow field of head guard, which consist of temperature, pressure and spatial distribution, are got by finite element simulation. On the basis of the numerical simulation, a method of which external flow field is equivalent to gas lens is proposed. The refractive index and focal length of the lens are calculated. The location change of focus of original infrared detection system is 49.66m.The effect on transmission of infrared lines can be neglected because the change is smaller than the focal depth. The difference between the MTF of degraded images got in wind tunnel test and infrared detection systems small, which verifies the validity of mechanism analysis of aero-optical transmission effects.