The invention relates to a method for machining threads by using a numerical control lathe and a thread machining tool thereof; wherein, the method comprises the following steps: (1) a thread machining tool is adopted and clamped on the cutter head of the numerical control lathe, (2) the sleeve of the tool is adhered to the tool body axially and tightly before machining the thread with a thread pitch of T, (3) during the machining, the axial displacement amount of the cutter head is 88 to 98% T / cycle, (4) the thread machining tool comprises a thread machining cutter, a tool body and a sleeve, and (5) the tool body is sleeve-jointed on the outer flank of the sleeve, the tool body and the sleeve can move correspondingly and axially, and the thread machining cutter is clamped on the front end of the sleeve. The machining method has the advantages of completing the screw thread machining of a plurality of thin and long workpieces continuously in one clamping, automatic processing, high degree of automation, high machining quality and stability, less times of cutter loading, low auxiliary time and machining time, high work efficiency, low machining cost, and low labor intensity of operators.