An underlaying device includes a main signal port, an expanding signal port, a signal process component, and a power connector. The main signal port is receiving and sending a communication signal from/to the computer device by means of a main signal wire. The expanding signal port is receiving and sending the communication signal from/to an external expanding device. The signal process component is coupled between the main signal port and the expanding signal port for transforming the communication signal into a signal which is able to be received and sent between the main signal port and the expanding signal port. The power connector is supplying power by means of a power wire. The underlaying device is suitable for various computer devices and is able to integrate the functionality of connection ports.