The invention provides a DC to DC converting circuit, comprising: a transforming unit with a primary winding and a secondary winding; a bridge rectifier unit coupled to an input voltage, having a first output terminal and a second output terminal coupled to both side of the primary winding respectively; a first switch coupled between the input voltage and the first output terminal; a second switch coupled between the first output terminal and a ground terminal; a third switch coupled between the input voltage and the second output terminal; and a fourth switch coupled between the second output terminal and the ground terminal; an output unit paralleled to the secondary winding; and a clamping unit coupled to the input voltage and paralleled to the bridge rectifier unit, having an auxiliary switch coupled to the input voltage; and a clamping capacitor coupled between the auxiliary switch and the ground terminal; wherein the auxiliary switch is turned on when operation statuses of the first switch and the fourth switch or the second switch and the third switch are changed.