1. A method for detecting distance in a smart portable device comprising: receiving a first digital image comprising the target object by a photo sensor of an image acquisition unit of the smart portable device; displaying and enlarging the first display image by a display unit of the image acquisition unit; selecting a digital image of the target object in the first digital image by a selection unit of the image acquisition unit; automatically calculating a length of a side of the target object in the digital image displayed on a display unit by a processing means of the smart portable device; acquiring a predicted length of the side of the target object from user prediction by the processing means; automatically determining a mil value of the length of the side in the digital image from the length of the side in the digital image by the processing means without user assistance; and calculating the distance between the smart portable device and the target object according to a ratio of the predicted length to the mil value by the processing means.