Conventional buck power factor correction(PFC) converters suffer from the drawback of high harmonic current, making it challenging to comply with the IEC61000-3-2 Class C limits and limiting their applications. This paper presents and analyzes a single-switch integrated buck PFC converter. The converter is composed of a buck and flyback PFC converter in an input-parallel-output-parallel connection structure. Only one GaN switch and its control circuit are used, reducing the converter’s cost. With critical conduction mode constant-on-time control, the effect of the dead zone of the buck input current is eliminated by the flyback unit. Taking advantages of the buck and flyback PFC converters and the excellent performance of the GaN switch, the converter achieves high efficiency and low total harmonic distortion over a wide voltage input range. This paper introduces the circuit structure and working principle. The theoretical derivations are validated using PSIM simulation software. Finally, a 120 W prototype with 94.1% peak efficiency is built to verify the analysis results.