Hydraulic ring-cutting device and method for use in pile head of prestressed pipe pile

The invention provides a hydraulic ring-cutting device and method for use in a pile head of a prestressed pipe pile. The hydraulic ring-cutting device comprises a telescopic positioning device and a cutting device; the cutting device extends into the pipe pile through the telescopic positioning device; the cutting device comprises a fixing mechanism and a cutting mechanism; the fixing mechanism comprises a cylindrical fixing block with a cylinder body provided with a plurality of movable outer supporting pieces; the fixing block internally comprises a horizontally-arranged rotating disc with a plurality of inclined grooves and a rotating groove allowing the rotating disc to rotate; the movable outer supporting pieces are movably connected with the rotating disc through the inclined grooves; each movable outer supporting piece comprises a horizontal sliding rod connected with the cylinder body in a sliding mode; one end of each horizontal sliding rod extends into the cylinder body, and the other end of each ho