The invention provides a reflecting surface folding and unfolding structure of a foldable and deployable antenna of space-based radar, belongs to the technical field of the space-based radar and solves the problems of existing foldable and deployable antennas. The reflecting surface folding and unfolding structure comprises a reflecting surface, transverse ribs, bistable composite foldable and deployable cylindrical shells and auxiliary unfolding inflatable arms, wherein the reflecting surface is rectangular, the multiple transverse ribs arranged in the width direction of the reflecting surface are mounted on the surface of one side of the reflecting surface, are bonded with the reflecting surface and adopt a hollow tubular structure, the surface, bonded with the reflecting surface, of each transverse rib is a plane, and the surface, opposite to the surface bonded with the reflecting surface, of each transverse rib is an indent arc surface; at least two bistable composite foldable and deployable cylindrical shells are arranged between every two adjacent transverse ribs, and two ends of each bistable composite foldable and deployable cylindrical shell are contacted with the adjacent transverse ribs respectively; the two transverse ribs located on the outmost side of all the transverse ribs are connected through the at least two auxiliary unfolding inflatable arms. The reflecting surface folding and unfolding structure is applied to the space-based radar.