The invention discloses a large-diameter folding optical device applicable to a spacecraft and a design method. The device comprises an instrument cabin, a primary mirror, a shading cylinder, secondary mirror support rods and a secondary mirror, wherein the instrument cabin, the primary mirror and the shading cylinder are sequentially connected; one end of each secondary mirror support rod sleevesthe shading cylinder, the other end of each secondary mirror support rod is connected with the secondary mirror, and the secondary mirror support rods can slide along the shading cylinder; the primary mirror comprises a central sub-mirror and a plurality of side-lobe sub-mirrors, the bottom end of each side-lobe sub-mirror is in pin shaft connection with the outer peripheral end of the central sub-mirror, and the folding directions of the every two adjacent side-lobe sub-mirrors are opposite; the side-lobe sub-mirrors are distributed along the outer periphery of the central sub-mirror. By thelarge-diameter folding optical device, the problems that an existing large-diameter folding optical device is large in size, is limited by a carrying space and is single in function, and an existinglarge spatial optical-mechanical system cannot perform automatic space correction easily are solved.