A transmission system includes a transmission apparatus, a first electronic device and a second electronic device electrically connected to one another. The first electronic includes a composite driver, a first application program, a first display image and a virtual display device. The first composite driver emulates the virtual display device according to the first display image. The second electronic device includes a second application program and a second display image. The transmission interface includes a transmission controller and two transmission interfaces. The two transmission interfaces are connected to the first electronic device and the second electronic device, respectively. When the first application program transmits output data corresponding to the virtual display device via the two transmission interfaces and the transmission controller to the second application program, a virtual display image corresponding to the first display image is presented on the second display image.