A primary mirror is a main imaging part of a space camera. Both its surface figure error and position error will decide the quality of imaging. A circular mirror assembly with an aperture of Φ660 mm is designed. It is restricted completely according to the six-point orientation principle. Its backside is supported by three fixers which are flexible multilayer frameworks. By reasonably distributing the freedom and rigidity of each supporting fixer and unloading the stress due to different expansion coefficients of different materials when temperature is changing, the distortion of the primary mirror assembly is symmetrical. The Finite Element Method (FEM) analysis result shows that the primary mirror assembly has a surface figure error (root-mean-square (RMS) value) up to 1/50λ (λ=632.8 nm), a first mode of 249 Hz and good dynamic rigidity.