In view of optical alignment of slit-lamp rotating drum, a new digital optical calibration method was presented. This method adopts the principle of transmission aligning system, adds the cross lines before and after the detecting optical path, judges the aligning quality by observing the imaging of two cross lines in the charge-coupled device (CCD) image and achieves the digital information through image processing at last. By extracting the midpoint coordinates of two cross lines, it can be judged whether the optical axis of the rotating drum optical system is consistent; by extracting the clarity of CCD image,it can be judged whether the rotating drum optical system is focusing . This method was used to align the rotating drum. The results show that the proposed method is a digital method to transform aligning information to digital information by using image processing technology, and its alignment error is less than 0.1 mm by digital image analysis.