In order to reduce influence of background infrared radiation, the temperature of the whole optical system should be below -20 °C for satellite-borne long-wave infrared imagers working in orbit. On the base of the weak heat conduction structure, a Ω type flexible sunshield made of MLI was developed and a cryogenic optical system was achieved through direct radiation cooling. Cage-like three-dimensional heat conduction straps made of copper were developed and an isothermal design for the body tube was realized. The cryogenic optical system applied to space remote sensing was used for the first time in China when it was tested in orbit with SJ-9B satellite. The results showed the temperature of the whole optical system could be maintained at -35 °C~-20 °C all the time, and the temperature difference in the body tube was no more than 4 °C. All flight test data met the temperature requirement of the long-wave infrared imager. This thermal control method is simple and effective, which can provide a reference for the thermal design of similar satellite-borne infrared optical systems.