The beam output of deformed Spaceborne SAR antenna array and the deformation error compensation are studied. Considering the flexible antenna array which may be adopted in the future Spaceborne SAR, a closed loop system for measuring and controlling of Spaceborne SAR antenna array’s deformation is proposed, and the steering vector model of deformed antenna array is presented. It was found that the mild deformation affect the beam’s sidelobe output more than the beam’s main-lobe. Then, the compensated weights were solved by the least square method, which can approximate the deformed antenna array outputs to the original antenna array outputs at most. Next, the deformed antenna array measuring precise demand was worked out. The validity of this method is verified by simulation.