Abstract: The characteristics of the working environment in the vacuum chamber determine that the collimator should be designed with the same thermal principles as the space optical remote sensor. A stringent temperature requirement is needed for the collimator. Firstly, the variety of thermal control coating of its structural surface and the coated mode of the multi-layer insulation was determined. Secondly, two schemes of active heating were setup. They were compared from the feasibility of implementation and the value of heating power. Finally, the selected scheme was simulated. It is shown that the temperature level of the collimator is 19.3-20.8℃, and the temperature level of the primary mirror is 19.5℃. It meets the design requirements and the thermal design is feasible. These methods could be a reference for the thermal design of other ground optical instruments.