A portable device includes a display module, an image capture module, and an eye-tracking module. The image capture module is used to capture images steadily. The eye-tracking module is used to track a user's viewpoint position relative to the portable device. Viewport within the images captured by the image capture module is adjusted in accordance with the user's viewpoint position for generating modified images displayed on the display module instantaneously. The modified images shown on the display module can fit the background scene seen by the user, and the augmented reality experience on the portable device may be improved accordingly.