An improved winding structure of a transformer, wherein the transformer comprises a winding base externally set with an isolating plate, the transformer is divided into a primary side region and a secondary side region by the isolating plate. The primary side region has a winding reel for a pre-formed wire set to sleeve on while the secondary side region is divided into a plurality of winding grooves by a plurality of partitions for placing a wire in the grooves, and an iron core set is set on the outside of the winding base and the hollow structure, which altogether form an transformer. In the present invention, an pre-formed wire set is sleeved on a primary side winding reel, therefore, the costs of traditional hand-winding will be displaced, and the manufacturing quality and usage stability of transformers will be effectively improved as well.