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 AC/DC 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) comparing a charging voltage with a reference voltage, thereby producing a high voltage level signal according to a comparison result, wherein the charging voltage come from the AC/DC adapter; and c) selecting a fast-charging mode on the secondary battery according to the high voltage level signal.