1. A method for marking electronic document, comprising: displaying an electronic document in a display unit; capturing an image sequence of a user by an image capturing unit; executing an eye tracking procedure, a nod detecting procedure and a blink detecting procedure to analyze the image sequence, wherein the eye tracking procedure is configured to track a movement trajectory of eyes of the user, and the nod detecting procedure and the blink detecting procedure are configured to detect whether the user performs a first specific action or a second specific action; obtaining a first position on the electronic document where a current sightline of the user falls through the eye tracking procedure when detecting the user performs the first specific action; executing a marking action for marking the electronic document along with the movement trajectory of the user's eyes starting from the first position; obtaining a second position on the electronic document where the current sightline of the user falls through the eye tracking procedure when detecting the user performs the second specific action; and finishing the marking action at the second position.