Detection range is an important evaluation index of infrared system application. Stray radiation is the main factor limiting the detection distance of infrared system, and the irradiance generated by it shows uneven distribution on the focal plane. Currently, the focal plane of the detector is regarded as a whole or the central region is extracted, and the influence factors such as self-thermal radiation and background radiation are calculated on average. The detection distance is obtained by inputting target parameters, and the overall influence of self-thermal radiation on the focal plane is considered in the cold optical design of the system. When the target imaging is in different focal plane regions, the detection range calculated by the above method is not accurate enough, and the pertinence is not strong in cold optical design. To solve the above problems, a detection range calculation formula including the system noise term is established, and a distributed detection distance analysis method is proposed, which is verified by a transmitted infrared optical system.Based on the self-thermal radiation and the classical detection range theory, this paper deduces the calculation formula of the detection range of the infrared system including the system noise term, and proposes an analysis method of the distributed detection range. Taking the transmitted optical system as an example, the sensitivity analysis of the influencing factors is carried out. By sub-regional data processing on the detector focal plane, the main influence surfaces corresponding to the detection range are obtained. On this basis, the change of the detection range before and after the low temperature treatment of the main affected surface (cooling from 293.15 K to 173.15 K) was analyzed （Fig.10, 11）.Based on the theory of self-thermal radiation and classical detection range theory, a calculation model of detection ability including its own thermal radiation noise is given, and a direct theoretical calculation relationship is established for the influence of the self-thermal radiation on detection ability. When the target imaging is in different focal plane regions, the detection range obtained through traditional calculation is not accurate enough and the pertinence is not strong in cold optical design, and the distributed detection range analysis method is proposed (Fig.1). Under the condition of only considering its self-thermal radiation, a simple partitioning principle is discussed (Fig.2). Taking the transmitted optical system as an example, the main influence surfaces in each area of the focal plane are obtained (Fig.9) through the statistical results of the influence weights of each component's own thermal radiation in different areas (Tab.3). The detection distance of the corresponding main influence region is significantly improved (Fig.10, 11), providing a new idea for the calculation of target detection range.Based on the theory of self-thermal radiation and classical detection range theory, the detection range formula directly related to the noise item of infrared optical machine system is obtained, the analysis method of distributed detection distance is proposed, and the principle of focal plane partitioning under the influence of self-heat radiation is given, and a transmitted infrared optical system is analyzed with this method. Under the premise that only the influence of self-thermal radiation is considered and the target is an ideal point target, the variation trend of detection distance along with the irradiance of image plane is given. Then, the focal plane of detector is divided into regions to obtain the irradiance ratio of different regions. Through the radiation amount generated by the surface light source of different components in different focal plane areas, the influence weight of each component self-thermal radiation in different areas was calculated, and the main influence surface of each area was obtained. On this basis, lens 7 and lens 3 were respectively treated with low temperature (293.15 K cooling to 173.15 K), and the maximum increase of detection distance in the main affected areas was 17.03% and 43.32%, with obvious improvement. It can be seen through the example verification that the proposed distributed detection range analysis method can be used as the basis for the calculation of distributed detection range and the design of cold optical index of infrared system after determining the corresponding partition principle according to the analysis environment and analysis conditions.