An external graphics method for an external graphics system is provided. The external graphics system includes a host device at least having a first graphics processing unit that generates a first video stream signal, and an external graphics device for connecting to the host device. The external graphics device at least includes a second graphics processing unit that generates a second video stream signal, and a TBT interface unit electrically connected to the host device. The method mainly includes: when the host device determining a graphics processing performed by the second graphics processing unit not exceeding a load-carrying capacity threshold, displaying an image frame corresponding to a TBT signal; and when determining the graphics processing performed by the second graphics processing unit exceeding the load-carrying capacity threshold, computing the TBT signal and the first video stream signal in sum, and displaying the corresponding image frame accordingly.