The large plain benchmark instrument is indispensable to assemblage of the large caliber space camera optical system, and the plain benchmark instrument’s caliber is required to be bigger and bigger because the caliber of camera optical system is bigger and bigger. In order to meet the requirements of the integrated precision of the mirror segment used for big space camera, selection of the materials of reflected mirror and supporting structure was introduced, and the design method of the flexible supporting structure of the mirror was proposed, which is analyzed and optimized through CAD/CAE engineering analysis software. Furthermore, a sort of reasonable flexible supporting structure for reflected mirror is optimized by means of Finite Element Method (FEM).