A combined transformer is provided. The transformer comprises at least three bobbins arranged abreast and a core assembly. Each of the bobbins includes two separated guard plates, a winding column, a through groove and two metal pins; the winding column is disposed between the guard plates, while the through groove extends through the guard plates and the winding column. Furthermore, the two metal pins are disposed on one of the guard plates; the winding column is wound with a coil, and two end portions of the coil are connected to the two metal pins respectively. The core assembly includes two separated magnetic plates and at least three separated magnetic columns disposed between the magnetic plates. The bobbins are sandwiched between the magnetic plates, and the magnetic columns are located in the through grooves. Thus, the combined transformer can have a reduced thickness and multiple outputs.