An asymmetrical planar transformer having controllable leakage inductance is provided. The asymmetrical planar transformer includes a circuit board, a winding rack, a primary winding, a secondary winding, and a magnetic core assembly. Via hole is formed on circuit board. Winding rack includes several annular plates, a tubular shell, and a through hole. The first, second, and third annular plates are arranged in parallel. Position of through hole is corresponding to that of via hole. Primary winding is disposed between first and second annular plates, and is electrically connected with circuit board. Secondary winding, with annular shape, is disposed between second annular plate and third annular plate, and is electrically connected with circuit board. Magnetic core assembly includes magnetic column which is passed through via hole of circuit board and through hole of winding rack. Through hole is extended through secondary winding, first annular plate, second annular plate, and third annular plate.