Abstract: The three main optical payloads of a scientific satellite need to achieve parallel view with the sun, which requires the parallelism deviation of their optical axes to be less than 30ʺ. Considering the influence from thermal elastic deformation, the temperature of the optical payload assembly platform needs to be controlled at (22±5) ℃. However, the payload assembly platform is large in size (1.6 m×1.6 m×0.8 m) and is located outside the cabin, resulting in great differences of heat fluxes in various directions and difficulties in temperature uniformity control. Therefore, combined thermal control measures of active PI closed-loop heating together with passive multi-layer insulation were implemented on the platform. Under the proposed thermal control scheme, software simulation shows that the temperature of the payload mounting plate and the support rod can be controlled at (22±3) ℃. Satellite vacuum thermal test indicates that the temperature of the payload mounting plate and the support rod can be controlled at (22±2) ℃ and (22±3) ℃, respectively. All the above have verified that the proposed thermal control scheme is feasible and effective.