An active clamping circuit. The active clamping circuit is applicable to a DC-to-DC conversion circuit, and has an output terminal to supply an output voltage to a load. In the active clamping circuit, a determining circuit is coupled to the DC-to-DC conversion circuit to determine the output detects the output voltage and to output a first enable signal when the output voltage is higher than a first predetermined voltage. A voltage adjustment circuit is coupled to the determining circuit to pull low the output voltage according to the first enable signal. An inductor has a first end coupled to the output terminal of the DC-to-DC conversion circuit, and a diode is coupled between the inductor and an input terminal of the DC-to-DC conversion circuit as a conductive path to channel discharge current to the input terminal of the DC-to-DC conversion circuit.