In order to make the space camera to work under the condition of appropriate temperature, the active thermal control system based on temperature level was proposed. Firstly, the condition of temperature balance was analyzed and the active thermal control strategy was ascertained. Secondly, the system composition and work principle were introduced. Thirdly, the temperature control indexes of active thermal control were determined under the condition of no essential influence to image quality. Finally, the algorithm of temperature control was determined. Experimental results indicate that the active thermal control system can make the camera in temperature balance based on the active thermal control strategy, the difference in temperature between primary mirror and reference temperature is no more than 1 ℃, the circumferential difference in temperature of primary mirror is no more than 1 ℃, the difference in temperature between primary mirror and secondary mirror is no more than 2 ℃ and the difference in temperature between primary mirror and third mirror is no more than 5 ℃, the results of temperature control are satisfied the requirements of control indexes of the active thermal control strategy and the camera work temperature.