Abstract: The antenna array is a critical component of a certain spaceborne SAR system, whose overall quality is directly determined by the performance of the antenna array. A major challenge in developing such an antenna array lies in achieving a lightweight structure while simultaneously ensuring high stiffness, high strength, and excellent thermal stability. This paper focuses on the antenna array of a specific spaceborne SAR system. First, the overall layout design is introduced, with detailed descriptions of the antenna modular design, stowed and deployed design, and the layout design of individual components. Subsequently, the structural design of the primary subsystems is elaborated, reflecting the design philosophy of lightweight construction, high stiffness and strength, and superior thermal stability. Finally, mechanical simulation calculations are conducted to verify that the structural stiffness and strength of the antenna array meet the mechanical environment requirements during the satellite’s main launch phase, with sufficient safety margins remaining..