To guarantee the image quality of a high-altitude camera in extreme low temperatures, thermal design for camera optical system was done by combined passive thermal control with active thermal control. Polyimide insulation material is used for passive thermal control, increasing the camera's thermal resistance between internal structures and external environment, and decreasing the influence of the external cryogenic environment on the temperatures of camera lens. The method of using electric heating film to heat camera lens was adopted as an active thermal control means. The heat transfer model of the lens in the optical system was built in the WorkBench finite element software with heating power loads, thermal convection loads and thermal radiation loads considered, and steady-state thermal analysis was carried out. It turned out that, the temperature of the camera optical system was controlled in the range of 18℃~22℃ when the heating power of six heating zones was 12、17、22、17、10、13 W, and the thermal control result meted the design requirements.