A PCI-based interfacing device with mappable port addresses to legacy I/O port addresses has an addressing circuit, a PCI controller connected to the addressing circuit and a PCI port, and an equipment controller connected to the PCI controller and an equipment port. The addressing circuit sets up a legacy I/O port address. The PCI controller transmit and receive data packets having data and one of the set of legacy I/O port addresses encapsulated therein to be processed to and from the PCI port, and output the received data packets to the equipment controller. The equipment controller converts the data packets into equipment data corresponding to the equipment port, and transmits the equipment data to the equipment port. Accordingly, the PCI-based interfacing device can perform data communication with legacy I/O port addresses.