Underground milling device

The utility model discloses an underground milling device which is provided with a body, an expanding body is coaxially attached in the body, and a sealing body is coaxially embedded in one end of the expanding body; a milling assembly is embedded in the body at the other end of the expanding body; the sealing body controls the flow direction of the high-pressure fluid in the expanding body, so that the axial moving direction of the expanding body in the body is controlled, and then the milling assembly is controlled to expand or contract in the radial direction relative to the body. According to the device, forward and reverse movement of the piston rod is hydraulically controlled so as to control retraction and release of the cutter body, compared with the prior art, the risk is reduced, the requirements of small wellhead drift diameter and underground large drift diameter milling are met, and the requirement of a large drift diameter patching pipe expansion technology for cleaning and shaping the inner dia