Abstract: A high-efficiency power supply transformer improved on its iron core which is a closed annular member made by continuous winding a silicon steel sheet and completed by sintering and fixing to keep directionality, after insulation processing with a heat shrinking wrapper on the core, a primary coil is formed by winding a wire annularly, then is enveloped by second time insulation processing with a heat shrinking wrapper; then a secondary coil is formed by winding a wire annularly and is enveloped by third time insulation processing with a heat shrinking wrapper; and pairs of end input wires of the primary and of the secondary coils extend outwards. With the core in the form of a closed annular member, its magnetic circuit is a closed magnetic circuit; hence its magnetizing energy and core consumption are 25% less than those of a conventional stacking type core and largely increase the efficiency of the power supply.