The invention relates to a buoyancy supporting device for transversely cutting an ultra-thin plate glass. The buoyancy supporting device comprises a transverse cutting machine (1), a supporting roller frame (2) and supporting rollers, and is characterized in that an air inlet main pipe (9) is arranged on the lower side of the supporting roller frame (2) and connected with a regulating valve (8) and a group of air inlet pipes (4); an air outlet panel (6) is arranged on the upper side of the supporting roller frame (2), an air outlet chamber (11) is arranged below the air outlet panel (6), the upper port of the air outlet chamber (11) is connected with the air outlet panel (6), and the lower port of the air outlet chamber is respectively connected with air outlets (10). By adopting the technical scheme, when an ultra-thin plate glass belt is cut, the plate glass belt is allowed to float on the air outlet panel by high pressure air, the downward pressure of a cutting head on the plate glass belt is resisted, and the plate glass is prevented from being fractured.