In order to solve the problem that when installing the slit lamp in the rotary drum, the human eye has an uncertainty of calculating the center distance of the cross-hair in the drum image, a digital adjustment technique based on the improved probabilistic Hough transform was proposed. By preprocessing the original image, improving the probability Hough line detection, and formulating the line intersection screening rules, the high-precision drum image cross-hair straight line detection and cross-hair center distance calculation were realized. Experiments show that the improved probability Hough transform can accurately detect the crosshairs in the image and fit the excess straight line 100% into 4 straight lines; and the intersection point screening rule can be used to accurately screen out 2 effective intersection points. The adjustment technology can realize digital assembly of rotary drum and the digital information can be used to remind the workers of the adjustment of drums better or, which can meet the actual needs of the drum production line.