A method for automatically adjusting an autostereoscopic three-dimensional (3D) display device is provided. The method includes steps of: displaying an autostereoscopic 3D frame by the autostereoscopic 3D display device; capturing a left-eye frame and a right-eye frame of the autostereoscopic 3D frame by two image capturing devices located at a left-eye position and a right-eye position, respectively; selecting multiple sampling points from the left-eye and right-eye frames; and analyzing the number of the sampling points satisfying a predetermined condition to determine a display angle.