Direct hydraulic cylinder type offshore drilling riser tensioning system

The invention discloses a direct hydraulic cylinder type offshore drilling riser tensioning system. A tensioner fixing frame is fixed on the lower surface of a floating drilling platform; a riser system of an underwater wellhead device is fixedly provided with a tensioning ring, and the tensioner fixing frame is connected with the tensioning ring through a hydraulic cylinder assembly; an accumulator, a high pressure air source, a control device and a control support assembly are arranged on a deck of the floating drilling platform; the accumulator is communicated with the hydraulic cylinder assembly through a recoil valve, and the high pressure air source is connected with the accumulator; and the recoil valve is connected with the control device. The control system is simple in structure, safe and reliable, and can provide approximately constant huge tension for an offshore drilling riser system in the floating drilling platform, thereby providing effective safety guarantee in drilling operation of the offshore floating drilling platform.