A power conversion system includes a power conversion circuit and a start-up circuit. The power conversion circuit includes an output capacitor, a first switching unit, a second switching unit, a flying capacitor and a magnetic element. The second switching unit includes two switch groups. The flying capacitor is connected between a first terminal and a second terminal of the power conversion circuit. The magnetic element includes two first windings that are electromagnetically coupled with each other. A first one of the two first windings is electrically connected between one switch group and the second terminal of the power conversion circuit. A second one of the two first windings is electrically connected between the other switch group and the second terminal of the power conversion circuit. The start-up circuit includes a third winding and an inductor. The third winding is electromagnetically coupled with the first windings.