The present invention provides a charging mode control circuit and method. By utilizing such charging mode control circuit and method, a secondary battery fixed in a portable device can be quickly charged when the portable device communicate with an external computer. The method includes the steps of: a) providing a commutator, the commutator comprising an adapter, a first interface, and a second interface, wherein the first interface is connected to the external computer and the second interface is connected to the portable device; b) filtering a charging voltage from the adapter and obtaining a ripple voltage from the charging power supply; c) rectifying the ripple voltage; d) comparing the rectified ripple voltage with a reference voltage, thereby producing a voltage waveform according to a comparison result; and e) selecting a fast charging mode on the secondary battery according to the voltage waveform.