A power conversion system includes a power conversion circuit and a start circuit. The power conversion circuit includes a first terminal, a second terminal, an output capacitor, at least one switch unit, a flying capacitor and a magnetic element. The flying capacitor is connected between the first terminal and the second terminal. The output capacitor is electrically connected with the first terminal or the second terminal. The start circuit is configured to control the power conversion circuit to pre-charge. A first terminal of the start circuit is electrically connected with the first terminal, and a second terminal of the start circuit is electrically connected with the second terminal. During a start process of the power conversion circuit, the at least one flying capacitor and the output capacitor are pre-charged by the start circuit.