A support device for the rotation of a high-capacity and compact drill pipe storage unit of a seabed drilling rig in a lying state, including: a rack, a plurality of curved support frames, a plurality of lateral drill pipe stoppers and a plurality of annular fences. The curved support frames are provided parallelly and coaxially on a slideway of the rack. A separating plate is provided on each of the curved support frames. A curved support surface of each of the curved support frames is in contact with a circular edge surface of the corresponding separating plate. The annular fences are mounted on the rack. Each of the annular fences is provided with a first gap and a second gap. The first gap is provided corresponding to the lateral drill pipe stopper. The second gap is configured to allow the drill pipes enter and exit the drill pipe storage unit.