A calculating method for the permissible temperature tolerance of the space telescope is studied and the temperature fields including every temperature state that the telescope is on orbit are described. The temperature field and thermal elastics based on a coupling model of the space telescope are analysized to abtain the displacement of optics surface and then its corresponding deformation is fitted as Zernike polynomial to use in a optical system. The MTF of telescope is calculated by transforming the coefficient of the polynomial into software CODE V. Experiment results show that when the MTF meets to the optical target

the ultimate temperature tolerance of the space telescope can be obtained

which is the aim temperature scope for the thermal design.