A connecting device having a built-in digital television tuner includes a base, a digital television tuner, a cover plate and a connector. The base has two ports. One of the two ports is assembled with the digital television tuner. The inner walls of both sides of the port have two symmetrical grooves. Rails provided on both sides of the cover plate are assembled in the grooves, so that the cover plate can slide in the port. The other port of the base is assembled with the connector. When being electrically connected to an electronic device, the connecting device of the present invention allows transmission lines of different functions to be inserted therein and receives digital television signals, whereby a user can watch digital television programs.