In order to effectively control the temperature of infrared detectors on carbon dioxide under complex heat flux, detailed thermal design was presented. According to the analysis of heat flux around the infrared detectors, positions of heat dissipation surfaces were confirmed. Based on the space thermal environment and the characteristics of complex heat flux, high power consumption and low thermal control index, the thermal solutions of infrared detectors were proposed. The thermal analytic calculation of the finite element model was carried out, and the temperature range of the infrared detectors was -31.8 ℃ to -26.9 ℃ in different postures of angle, the results met the design requirements. A thermal balance test of carbon dioxide was actualized to verify the thermal design, the temperature of the infrared detectors was from -32.6 ℃ to -30.1 ℃. These results conform with the calculated ones and meet the thermal controlling requirements, the thermal design is reasonable and adaptable in complex heat flux.