The invention discloses a planar transformer based on double-sided printed circuit boards. The planar transformer comprises magnetic cores and printed circuit boards, wherein the magnetic cores surround the peripheries of the printed circuit boards; pattern layers are arranged on the front side and the back side of each printed circuit board; each pattern layer comprises a coil zone, a lead node and a connecting point; the coil zone on the back side of the printed circuit board on the upper layer vertically coincides with the pattern layer on the front side of the printed circuit board on the lower layer, and the lead nodes on the front side and the back side of each printed circuit board are located on the upper side and the lower side or the left side and the right side of the corresponding printed circuit board; the planar transformer further comprises connecting assemblies for connecting different layers of printed circuit boards. A primary winding or a secondary winding has rich constitution modes, the number of layers of the printed circuit boards is reduced by a half under the condition of the same turn ratio, the thinning development of the planar transformer is facilitated, different turns ratios can be realized according to the actual demand, and the functions are diversified.