A composite transformer with a longer creepage distance includes a primary winding rack and an insulation support rack mounted onto the primary winding rack. The insulation support rack includes a first insulation half shell and a second insulation half shell that have respectively a first encasing portion and a second encasing portion to encase the primary winding rack. The first and second encasing portions have respectively a first insulation portion and a second insulation portion, and a first isolating portion and a second isolating portion extended respectively from the first and second insulation portions. The first isolating portion has a first covering section extended towards the second isolating portion to cover the second isolating portion. The second isolating portion has a second covering section extended towards the first isolating portion to cover the first isolating portion.