A new kind of active clamp single-stage PFC converter is presented which is combined by a PFC converter and an active clamp DC/DC converter. Its circuit operation principle was analyzed and the four working stages of the switch were defined. The circuit was simulated with Saber software, and the result shows that the circuit achieves zero voltage switch for principal and assistant switches and over 0.98 high power factor under rated conditions and improves the efficiency and power density of the converter.