Support system design for large size rectangular mirror is one of the most challenging technical points during development of off-axis three-mirror anastigmatism (TMA) optical imaging sensor. The rectangular third mirror with the size of 760 mm×320 mm is the study object. Key mechanism of the temperature effect on the mirror′ s optical surface deformation is studied thoroughly. The relationships between temperature range and the mirror blank′ s and back support part′ s linear expansion coefficient difference, mirror blank′ s capacity on interface mechanical load, rigidity of the flex structure which connects mirror blank and back support part, are established. During development of the mirror support system, a two-axis flex support structure which is composed of two perpendicular single axis circular flexures is proposed, and finite element analysis (FEA) is also employed to evaluate the mirror′s optical surface error and model frequency of this mirror sub-assembly. In the mean time, sine scan test is used to verify the FEA result of mirror sub-assembly′s model frequency. Under combination of 1g self gravity load and 10 ℃ temperature variation, the maximum FEA value of the mirror optical surface root-meansquare(RMS) error is 13.4 nm, and it meets the proposed requirement λ 45 ( λ = 632 nm ). FEA result and test value of the lowest model frequency of the mirror sub-assembly are 128.67 Hz and 124.1 Hz, respectively, it is higher than the proposed requirement of 100 Hz and meets design objective.