An electronic device and an associated method for displaying image data on a first display device of a first electronic device and a second display device of a second electronic device external of the first electronic device are provided. The method includes the steps of: determining whether image data to be displayed on the first and second display devices are the same; when the image data to be displayed on the first and second display devices are the same, estimating the first resource consumption required when the extension mode is selected for displaying the image data and the second resource consumption required when the mirror mode is selected for displaying the image data; and determining to display the image data on the first and second display devices in the extension mode or the mirror mode according to the first and second estimated resource consumption.