The invention discloses an inner support device for processing an external thread of a pipe. The inner support device comprises a top wire, a fixed disk body, a limiting sleeve, a wedge, a cone and asupporting body, wherein the fixed disk body is connected with the limit sleeve through a fastening bolt and a nut, the cone is located in the limiting sleeve, a rectangular hole is formed in the sleeve wall of the limiting sleeve, one end of the wedge is provided with a slope, the wedge is inserted into the rectangular hole and is in sliding fit with a slope of the cone, the other end of the wedge is connected with the supporting body, the curvature of the upper surface of the supporting body is the same as the curvature of the inner wall of the pipe, the top wire is screwed with the fixed disk body, the wedge and the supporting body move up and down along the rectangular hole of the limiting sleeve by adjusting the elongation of the top wire, and when the supporting body moves upwards, the supporting body is contacted with the wall of the pipe, so as to form contact force. The device can prevent the pipe from being deformed elastically due to external clamping force during processingof a sample with limit-size external thread of an oil casing pipe, so that the pass rate of processing the sample with the external thread with the limit size is improved.