The invention discloses an optimum design method for a buttress thread joint incomplete thread part. The method comprises the steps that firstly, a buttress male thread and a buttress female thread which are in 1:16 taper are machined according to a conventional thread machining method; secondly, conventional deburring treatment is conducted on the buttress female thread; thirdly, a dedicated special tool is used for conducting thread turning machining again on the buttress male thread machined in the first step according to the cylinder thread machining track at the other tool installing position under the condition that the thread phase position is kept unchangeable; and thirdly, the dedicated special tool comprises a body and a tool edge which is arranged on the body and makes contact with the top of the buttress male thread, the tool edge is a symmetric arc opening, and the distance between radians R of the opening is the half thread pitch. The optimum design method has the beneficial effects that a male thread part at the pipe end cannot be polished, the circular bead of a tooth crest of the incomplete thread is machined through a machining method, and burrs are avoided.