1. A method for identifying dimensions of a shot subject, implemented on an identification system having a photo shooting unit capable of adjusting focal lengths, the method comprising: using the photo shooting unit to focus on plural positions respectively having different field depths on a shot subject and capture plural images of the focused positions on the shot subject; comparing and determining whether resolutions of the captured images are same or not; and when the resolutions of the captured images are the same, the shot subject is a two dimensional object; when the resolutions of the captured images are not the same, the shot subject is a three dimensional object.