The invention provides a secure digital card. In one embodiment, the secure digital card is coupled to a cell phone, and comprises a smart card, a secure digital card controller (SDC), a first power control circuit, and a second power control circuit. When the cell phone wants to perform a near-end payment process, the SDC controls the first power control circuit to supply a first voltage generated by a first power supply pin of a near field communication (NFC) controller to a power receive pin of the smart card. When the cell phone wants to perform a far-end payment process, the SDC controls a second power control circuit to supply a second voltage generated by a second power supply pin of the SDC to the power receive pin of the smart card.