A power conversion system includes an isolating transformer (30) and an output-controlling device (40). The isolating transformer (30) includes a primary winding (310) and a plurality of secondary windings (320a˜320d) coupled with the primary winding (310), and the isolating transformer (30) has a plurality of coupling distances between the secondary windings (320a˜320d). The output-controlling device (40) includes a controller (420) and a plurality of output-controlling modules (400a˜400d), wherein each one of the secondary windings (320a˜320d) is electrically connected to one of the output-controlling device (400a˜400d). The controller (420) places at least one output-controlling device (400a˜400d) in a conducting state for output a rectified power. An amount of the secondary windings (320a˜320d) coupled with the primary winding (310) is modulated for varying a leakage inductance of the power conversion system.