A power converter, a synchronous power converter system and a method of determining switching frequency are provided. A processor is configured to output a synchronous clock signal corresponding to a first switching frequency. A plurality of first-stage power converters are coupled to the processor, and configured to generate a plurality of first output voltages corresponding to the first switching frequency according to the synchronous clock signal and a system voltage. At least one second-stage power converter is coupled to the processor and one of the plurality of first-stage power converters, and configured to generate a second output voltage corresponding to a second switching frequency according to the synchronous clock signal, a multiplied frequency control signal and one of the plurality of first output voltages. The second switching frequency is a multiple of the first switching frequency.