Abstract: The large space remote sensing instrument has high requirements for heating and mechanical characteristics, as well as strict limitation in weight, to adapt to severe launching and working environment. Thus the mirror and supporting system is stringent in weight, rigidity and environmental flexibility. The paper analyzed, designed and demonstrated the parameters of a 620 mm remote sensing mirror and its mounting structure. The central mounting system was served to meet the light weight and rigidity requirements compared with the other mounting methods. Radical weight-reduction slot morphology was developed in accordance with equal strength principle. The partial contact stress would increase as temperature changed. A flexible material was utilized to solve the problem, which would mitigate the structure effect to optical mirror. The results show that the weight of mirror is 15 kg and mounting accessory is 3.5 kg, with the weight reduction rate of 71.2%. The first mode is 420 Hz;The profile accuracy of mirror is 0.03 RMS. The technical parameters are stable and satisfied with the optical system requirements under various tests. The primary mirror project is implemented in the space and attains the expected goal.