A multi-port power supply device and an operation method thereof are provided. The multi-port power supply device includes multiple USB ports, multiple power converters, and a common control circuit. When an adjustment trend of an agreement power of a first USB port will make the agreement power greater than a rated minimum charging power of a USB device connected to the first USB port, the common control circuit dynamically changes the agreement power according to an actual output power of the first USB port. When the adjustment trend of the agreement power will make the agreement power less than the rated minimum charging power, the common control circuit does not change the agreement power, and the common control circuit dynamically transfers a power difference between the agreement power and the actual output power to other USB ports.