The proposed Chinese Space Solar Telescope (SST) is the first large aperture space telescope in China designed to observe the sun. With an effective aperture of Φ1m, the primary mirror faces the sun directly, which receives more than 1000W heat that will lead to unacceptable thermal distortion in such severe thermal condition. Therefore, the temperature field of SST, which is changing with its orbital position, is critical in its design. In this paper, an analysis of the thermal flux in the SST is presented firstly. Further more, the heat flux of orbit is calculated with the thermal softerware NEVADA (Net Energy Verification And Determination Analyzer) according to the orbit parameters of SST. The thermal design software SINDA/G (System Improved Numerical Differencing Analyzer/Gaski), the radiation analysis software NEVADA and the finite element analysis software MSC.Patran are used to simulate the temperature field of the SST. In the end, the temperature distribution of the primary mirror is calculated. The temperature level of the primary mirror indicates that the system can achieve high spatial resolution with 0.1″~0.15″. It also means that the thermal control design is effective. The optical requirements to the SST thermal control are met. The thoughts and methods of the thermal analysis are also useful for similar optical telescopes designed for solar observation.