The invention discloses a riser support device under ultra-deep sea water. The device comprises a riser support buoy, a tension mooring device and an umbilical cable, wherein a wellhead A is formed on the upper part of the riser support buoy; a channel which passes through the riser support buoy is formed on the lower part of the wellhead A; and the riser support buoy has a starfish structure, namely the edge of a central post structure buoy is connected with three rectangular section cantilever buoys which form included angles of 120 degrees on the plane. The tail end of each cantilever buoyhas a truss structure with neutral buoyancy, so that the buoyancy requirement of the riser support buoy can be met by reducing or increasing the length of the truss structure on each cantilever buoy,the construction process difficulty of each cantilever buoy can be greatly reduced, and construction cost is reduced. The riser support buoy provides tension for a top tension riser (TTR), so that a dynamic load supported by a water surface floating production device is reduced, and the requirement on the water surface floating production device is reduced; therefore, the construction cost of thewater surface floating production device is reduced.