The invention relates to a production method for a variable-diameter plastic three-way joint suitable for large water supply and drainage pipelines and industrial pipelines and relates to small-caliber branch pipes generated on large-caliber main pipes in a branched mode. Firstly, a plastic main pipe is machined to specified length, and a small-caliber side through hole in the side wall of a pipe body of the plastic main pipe is machined; the connection end face of an injection molding prefabricated saddle-shaped small-caliber branch pipe is provided with a cambered surface with the same curvature of the outer diameter of the main pipe through machining, and electrical heating wires are arranged on the cambered surface spirally in an inlaid mode; and then an inner hole of the saddle-shaped branch pipe is aligned with the small-caliber side through hole of the main pipe, thus connection hook faces are matched, and the main pipe and the saddle-shaped branch pipe are welded and integrated through heating of the electrical heating wires. The production method has the beneficial effects that the whole bearing ability of the large-caliber welded saddle-shaped three-way joint is greatly promoted compared with an existing machining mode, and the whole size of a structural part is shortened. In underground construction, earth excavation can be reduced, thus construction cost is reduced, and pipeline installation space is saved. Repeated butt welding of the branch pipe during machining of the large-caliber variable-diameter three-way joint is reduced, quality hidden danger is reduced, and the production cost is saved.