A method for providing a video playback device with a source selection function includes: providing the video playback device with a user interface of the source selection function for displaying image representatives of a plurality of input terminals of the video playback device at the same time; and selecting a signal corresponding to a specific input terminal of the input terminals in response to user selection, in order to make images carried by the selected signal be displayed. An associated video playback device and an associated processing circuit are provided. In particular, the processing circuit includes: a controller arranged to control operations of the video playback device; and a program code embedded in the controller or received from outside the controller, where the controller executing the program code is arranged to provide the user interface of the source selection function for displaying the image representatives at the same time.