An auto focus device comprises a focus panel and a focus controller. The focus panel comprises liquid crystal between a first light-transmissive conductive film and a second light-transmissive conductive film, and the focus controller is configured to apply a voltage between the two light-transmissive conductive films at the position of at least one pixel, so that the liquid crystal at the position will have an expected focal length. An auto focus method comprises: acquiring information on eyesight status of a user; acquiring information on posture of the user; calculating expected focal length of the liquid crystal between the two light-transmissive conductive films at the position of at least one pixel according to the acquired user information; selecting a voltage to be applied between the two light-transmissive conductive films at the position according to the expected focal length; and applying the voltage between the two light-transmissive conductive films at the position.