The width measurement of cracks by machine vision has problems of offsets in the width direction of the cracks and inaccurate measurements. To this end, taking the cracks of reservoir dams as objects, the method of crack backbone refining and width measurement was studied. Based on image thinning, the proposed method refined the cracks' backbone further. The total number of points in the eight neighborhoods of each backbone's point did not exceed two. Redundant data points were simplified, and the number of neighborhood distribution types was reduced to 16, which enhanced the backbone's ability to describe the crack shape. The backbone's macroscopic and microscopic features were combined as a basis for the width measurement direction to obtain a more accurate measurement direction than a baseline method used for comparison. As a result, the proposed method achieved continuous and accurate visual measurements of the crack width. By adding two evaluation criteria, measurement recall rate and direction error, the proposed method was validated to be more accurate in actual engineering requirements than the baseline method. The proposed method has practical engineering applications and can be a reference for radial vision measurement of other slender and irregular targets.