Abstract: Disclosed is a PCB of a planar transformer including: a PCB substrate with a through hole and a double-sided winding part formed on double sides of the PCB substrate, wherein the double-sided winding part is of a symmetric structure, a via hole consistent with the through hole is formed in the center of the double-sided winding part, the via hole is aligned to the through hole to form a magnetic core hole, and the circumference of the via hole is raised to form wire blocking parts; and forming a wire passing groove in the wall of the magnetic core hole, wherein the wire passing groove allows a metal conducting wire to pass through to be planarly wound from inside to outside on double sides of the double-sided winding part at the same time so as to form two coils in series located on the double sides of the PCB substrate.