Some space-based telescopes have primary mirrors with great size which make active thermal control almost impossible. Sunshield is introduced to provide a relatively stable environment. Primary mirror's diameter is commonly several meters large and several millimeters thick such as in NGST or JWST. Even after segmented, it still could be one meter large. If temperature difference exists between two sides of mirror commonly with no heat source inside, great deformation could happen and then decrease performance of whole system. Theories to analyze deformation considering radius, thickness and material properties of mirror based on simplified models are proposed. Primary mirror is supposed to be spherical. Considering axial symmetry, only part of primary mirror is analyzed. To be convenient, thermal stress is converted to a moment by select proper thermal refercence. Most deformation caused by thermal stress can be seen as impacts of equivalent moment at both ends of selected part. Based on such hypotheses, analysis can be greatly simplified. At last, relationship between deformation and radius, thickness or material properties of primary mirror is given. To testify results, thermal deformation cause by thermal gradient along radius is also calculated using finite element method software: MSC NASTRAN/PATRAN. Considering axial symmetry, only 1/4 part of the segmented primary mirror is modeled. Fitful boundary conditions and temperature loads are also applied to it. Finally results from theoretical and finite element method were coincident. Simplified theories were proved to be accurate.