The skein wire is used to replace the single hot wire, and the dumpling wire is twisted by a plurality of hot wires. The function of the rubber wire is mainly to effectively increase the diameter of the hot wire and expand the range of the hot wire. During the CVD deposition process, the stranded wire is straightened with a high temperature resistant spring and coincides with the axis of the inner bore of the mold. Thus, after the plurality of hot wires are twisted, the heat generation amount of the whole hot wire is increased, the diameter of the hot wire is increased, and the symmetry and uniformity are ensured, and a uniform diamond coating can still be obtained on the surface of the aperture. On the other hand, because the distance between the hot wire and the surface of the aperture is shortened, the active hydrogen atoms near the surface of the aperture are increased, and the temperature of the surface of the substrate can be maintained at a relatively high level. Therefore, the quality and thickness of the remaining layer can be guaranteed to meet the actual use requirements.