Drilling, milling and cutting device in underwater pipe and using method of drilling, milling and cutting device

The invention provides an underwater in-pipe drilling, milling and cutting device and a using method thereof.The device comprises a cutting device, a rotating device and a fixing device, and the fixing device is composed of a supporting hydraulic cylinder, a supporting short arm, an upper flange, a lower flange and a main structure steel pipe; the slewing device is composed of an outer gear slewing bearing, a driving gear meshed with the outer gear slewing bearing, a speed reducer, a hydraulic motor and a circular working platform. The cutting device is provided with two sets of identical mechanisms and is composed of a driving motor, a vertical milling head, a milling cutter, a feeding hydraulic cylinder and a sliding rail. And the two sets of cutting devices are oppositely mounted on the rotary working platform at an angle of 180 degrees. The lower flange of the fixing device is connected with the outer ring of the slewing bearing; the cutting device is connected with the inner ring of the slewing bearing t