The invention discloses a clamping device for boring machine processing of a supercritical steam turbine low-pressure last stage blade tiepiece boss, and relates to a clamping device for boring machine processing. The clamping device comprises a supporting body, a blade mounting plate, a first bolt, a second bolt and four third bolts; the supporting body is composed of a base plate, an L-shaped plate and a top plate; the top plate, the L-shaped plate and the base plate are sequentially arranged from top to bottom and are made into a whole; each side face of the base plate is provided with two U-shaped grooves, a side threaded through hole is formed in the plate face of one side of the L-shaped plate, a threaded through hole is formed in the top plate, the upper end face of the blade mounting plate is provided with an oblique line type blade root groove matched with the blade, and the blade mounting plate is arranged on the upper end face of the base plate and is located on the inner side of the L-shaped plate; the blade mounting plate and the base plate are connected through the four third bolts, the first bolt is in threaded connection with the upper threaded through hole, and the second bolt is in threaded connection with the side threaded through hole. The clamping device is used for machining the tiepiece boss on a supercritical steam turbine low-pressure last stage blade.