A power supply control method and a portable electronic device using the same are provided. The power supply control method includes following steps: detecting an input voltage and an input current at a power input terminal of the portable electronic device; setting a plurality of detection loads sequentially to control a power adaptor to provide a detection current as the input current for the portable electronic device respectively; calculating an equivalent input impedance of the power input terminal according to the detection current and the corresponding input voltage; calculating an actual output voltage of the power adaptor according to the equivalent input impedance, the input voltage, and the input current; and setting a work load according to the actual output voltage to control the power adaptor to provide a work current as the input current for the portable electronic device.