To improve the adaptive capacity of a space camera to environment temperatures and to enhance its imaging quality, this paper explores how to implement the thermal control by relaxing thermal control indictors. A mathematical formula of intercept and defocus affected by temperatures is derived for a pure off-axis Three Mirror Anastigmatism(TMA) optical system. The analysis shows that camera defocus amount and temperature are a linear relationship. Then, a constant K value is calculated. Furthermore, it indicates when reflector and camera structures have the same coefficient of linear expansion, the defocus does not occur. However, the greater the difference between two material linear expansion coefficients is, the greater the amount of defocus is. With defocusing formula, it suggests that mechanical focusing methods must be used to request for compensation when the camera temperature level changes more than ± 1 ℃(corresponding to the defocus amount of 0.05 mm). Using integrated photo-thermal simulation method , the temperature range Δt is obtained to ensure the image quality of the camera. Finally, camera′s focus range is derived by using the formula of defocus. The result of defocus test under thermal vacuum environment verifies the correctness of the defocus formula and the defocus amount of ± 0.184 mm is a proper focus range corresponding camera temperature levels changed in ± 4 ℃.