The embodiment of the invention discloses a direct current-direct current conversion circuit which can work at three levels on the premise of not generating common-mode current. The DC-DC conversion circuit comprises an input inductor, a first capacitor, a second capacitor, a flying capacitor, a first switch, a second switch, a first inductor, a first diode, a first buffer circuit, a third switch,a fourth switch, a second inductor, a second diode and a second buffer circuit, wherein the power supply, the input inductor, the first diode, the second diode, the first capacitor and the second capacitor are sequentially connected in series, a first end of the flying capacitor is connected between the first diode and the second diode, a second end of the flying capacitor is connected between the first switch and the third switch, and a second end of the flying capacitor is further connected between the second switch and the second inductor.