Abstract: In order to meet the requirements of long-life and high reliability of spaceborne synthetic aperture radar (SAR), the technical route of a large planar phased array antenna for spaceborne SAR was introduced from the aspects of antenna type selection, configuration design and implementation approach. The whole slot waveguide antenna using vacuum brazing was selected as the research object. The key technologies and difficult points of the adaptive design in mechanical, thermal radiation, high and low temperature, vacuum and space radiation environments were comprehensively analyzed from mechanical, electrical and thermal perspectives. For example, the rigidity and strength of the antenna were solved through the designs of reasonable weight reduction, structural partitioning and layout optimization. The anodized film that meets the thermal control requirement was obtained through the whole process control of the antenna. The free design was used to reduce the thermal deformation of the antenna at high and low temperature. By reasonably determining the spacing between the two metal surfaces at the strongest electric field, the multipactor in vacuum could be reduced. The anti-irradiation capability of the antenna was improved by selecting polyimide as the dielectric support of the RF connector. The results of environmental test and on-orbit verification show that the design of slot waveguide antenna is reasonable with good environmental adaptability, and is able to work stably and normally.