Radiators used to maintain space target functioning normal were introduced. Operating mode and working condition were presented. Current used space target infrared characteristics models were summarized. With further analysis, the outside surface of space target were divided into common area and radiator, and energy equations were built separately. Taking FY-1C as an example, and taking account of orbit, materials and structure, temperature field of the outer surface of space target was calculated using finite element method. When dissipation power was 0 W and 100 W, the biggest temperature difference of radiator was 51.49 ℃. Analyzing the temperature field with orbit, the illumination in entrance pupil of detection system in a distance of 5 km was calculated. When space target was in the shadow of earth, during which the radiation from earth or solar radiation reflected by earth was negligible because of the big incident angle, the illumination of the space target differed by 1-2 orders of magnitude. When space target was under the sun, different dissipation powers can only affect the illumination of long infrared spectra apparently, because of the reflected radiation of target.