A resonant transformer with adjustable leakage inductance includes a secondary winding group, a primary winding group, a magnetic sheet and a core group. The primary winding group is provided on a bobbin of the secondary winding group, and the magnetic sheet is provided in the bobbin. The secondary winding group, the primary winding group and the magnetic sheet include a first through hole, a second through hole and a through hole, respectively. The core group includes a first core and a second core symmetrically disposed, which are disposed on top of and at the bottom of the primary winding group, respectively. During operation of the resonant transformer, the degree of coupling between the primary and secondary sides can be changed using the magnetic sheet. This allows the native leakage inductance to be altered to satisfy demands for various different resonant frequencies.