A multi-mode active clamping power converter comprises an input inductor, a down-bridge switch, an up-bridge switch, a first energy-storing capacitor, a clamping capacitor, a resonant inductor, a magnetizing inductor, a transformer, an output diode and an output capacitor. The input inductor has a boundary inductance value, wherein an input voltage source is operated between a first voltage and a second voltage and the boundary inductance value is set according to the first voltage and a heavy load so as to be served as an initial condition of the multi-mode active clamping power converter, and then the input inductor is operated in a boundary conduction mode.