In order to satisfy the development requirement of spaceborne telescope primary mirror,a design of lightweighted primary mirror sub-assembly structure was presented.The influences of the material thermal math,lightweight pattern and the optimal support structures were considered during the mechanical/thermal design process.A high resolution Cassegrain optical payload with Φ650 mm primary mirror was designed,in which its specially support system could keep both high rigidity of structure and fine stability of thermal dimension.By considering the telescope manufacture and work station,the reasonable and optimal structure of the primary mirror sub-assembly was taken finally,analyzed results show its PV is less than λ/10,which can satisfy the requirement for the high resolution satellite primary mirror design.