The invention discloses an inner groove machining device serving as a lathe. The inner groove machining device comprises a clamping piece, a body, a groove depth control device, an axial positioning dimension control device and a clamping device, wherein a long rectangular convex strip is arranged on the left side of the clamping piece; the upper surface and the lower surface of the right side ofthe clamping piece are each provided with a rectangular guide rail convex strip in the front-back direction; guide rail grooves which are positioned in the front-back direction and are in sliding fitwith the rectangular guide rail convex strips are formed in the right side of the body; the groove depth control device is mounted on the left side of the body and rightwards passes through a first through hole and finally extends into a combined groove in the middle of the right side face of the clamping piece; the clamping device is mounted on the groove depth control device located on the leftside of the body; and the axial positioning dimension control device is mounted on the clamping device. The inner groove machining device disclosed by the invention has the advantages that the size control is convenient, the operation is convenient and fast, the machining precision can be ensured, and the machining efficiency can be improved.