An electronic paper display apparatus, a signal transmission system and a method thereof are provided. The electronic paper display apparatus is configured to transmit an encrypted signal. The electronic paper display apparatus includes an electronic paper display panel and a display driver. The electronic paper display panel is configured to display a first image or a second image. The display driver is electrically coupled to the electronic paper display panel and configured to output a driving signal to the electronic paper display panel. The display driver drives the electronic paper display panel to alternately display the first image and the second image for transmitting the encrypted signal by using the driving signal during a signal transmission period.