In order to evaluate the temperature effect of telescopes, a precise thermal finite element model consists of telescope optical assembly systems, mount structure and electrical equipment was established. Temporal and spatial temperature distributions of the optical assembly and telescope structure were calculated under various thermal conditions including conductions, air convections，radiation and heat flux loading. Moreover, the thermo-elastic analysis was made to obtain thermal deformations of the telescope structure based on the resulting temperature distributions. The obtained results show that temperature variation has large effect on telescope mount and main optical system, and induced displacement between primary and second mirror has a magnitude of millimeter, so it must be controlled strictly. At last some thermal control ways were given in terms of protection dome, mount, primary mirror and so on.