A series of mode experiments for the infrared signature suppression (IRSS) device used in the helicopter gas turbine exhaust system were carried out to investigate the effects of ambient air pumping-mixing and heat shelter-insulation on decreasing the target infrared signature and quest for the infrared signature similarity between two scaled models with different scale factors. The results show that (1) Ambient air pumping-mixing plays a dual role for reducing the exhaust temperature and mixing duct temperature and this role is more obvious in the case of larger mass flow ratio of secondary flow to primary flow. Sheltering from the hot mixing duct wall makes the sheltering sheath temperature close to ambient temperature in case of the sheltering distance greater than 20mm. (2) Ambient air pumping-mixing contributes about 85% suppression for the total target infrared radiation intensity and heat shelter-insulation contributes about 10% suppression again. (3) While the primary flow velocity, temperature and pressure are at the same condition intensity is almost proportion to the square of model scale factor.