A system for providing a bidirectional data access service includes a relay server and a service server. The relay server is located in the Internet. The service server registers an internal Internet Protocol (IP) address, an external IP address, and a corresponding server identification in the relay server, and establishes a data channel. When a user wants to utilize a predetermined service provided by the service server through a client device, the client device transmits the corresponding server identification to the relay server, and the relay server transmits the internal IP address and the external IP address to the client device according to the corresponding server identification. The client device automatically determines a connection method between the client device and the service server according to an application program, the internal IP address, the external IP address, and a network on which the client device is located.