1. A method for actively detecting and recognizing a placard, the method comprising: moving an image capturing device according to a maneuver rule, wherein the image capturing device continuously captures an image during the movement; determining whether a placard exists in the image; identifying a content of the placard and executing an action according to the content if the placard exists in the image; and repeating the foregoing steps to continuously move the image capturing device according to the maneuver rule and determining whether the placard exists in the image newly captured.