Abstract: To meet the thermal control requirements of the main structure and optical components of a large-aperture off-axis three-mirror space telescope in GEO, a thermal control scheme combining overall thermal control with precision heating at key positions is proposed. The scheme integrates passive thermal protection with multi-layer insulation (MLI), surface blackening, localized heat flux shielding, and active heating, along with 112 low-power heaters and three precision temperature controllers. Thermal analysis under typical operating conditions was conducted using thermal simulation software, and thermal balance tests were carried out in a vacuum chamber for validation. The results showed that the maximum temperature difference of the main structure was 0.4 ℃, with a temperature stability of less than 0.1 ℃ over one orbital period. For the optical components, the maximum temperature difference was 0.1 ℃, and the temperature stability was also less than 0.1 ℃ over one orbital period. All thermal control parameters meet or exceed the design requirements. This thermal control scheme can serve as a reference for the thermal design of similar high-precision space optical payloads.