The present invention provides a transformer having assembled bobbins and a voltage transformation module having the transformer. The transformer includes a base, bobbins, secondary windings and two magnetic cores. The base is provided with a penetration hole. The bobbins are disposed in the base and each has an annular groove, a hollow portion corresponding to the penetration hole, and protrusions formed on a surface of the bobbin. The protrusions form a gap between the two adjacent bobbins when the two adjacent bobbins are assembled with each other. The secondary windings are disposed between the bobbins and each has a through-hole corresponding to the hollow portion. The two magnetic cores penetrate the penetration hole of the base, the hollow portions of the bobbins, and the through-holes of the secondary windings to assemble them together.