Estimation of the detection distance of an infrared imaging system is of great importance to the design level improvement,cost reduction and development and production period shortening for infrared systems.However,the detection distance can be affected by many factors such as the characteristics of a target,the transmission in atmosphere,the parameters of the thermal imaging system and etc..These make the estimation of the detection distance of an infrared imaging system be very difficult.The traditional estimation method of the infrared detection distance generally considers the atmospheric transmission coefficient as a constant.This will result in a great error.In this paper,the variation of the atmospheric transmission coefficient with the environment in the atmospheric aerosol mode is considered and a more detailed expression of the detection distance from a target is given.By programming and searching for the relevant data,the detection distances of an infrared imaging system against a point source and a extended source are calculated.