Abstract: To address the problems of automatic localization, width quantification in all directions and extended tracking of shear diagonal cracks in reinforced concrete beam experiments, this paper proposes a computational method for automatic crack extraction and width quantification based on digital image correlation (DIC) technology. The horizontal, vertical and shear strain fields calculated by DIC are used to calculate the principal tensile strain fields on specimen surface, and the part of the specimen surface where the principal strain is greater than a certain threshold value is considered as cracked. Morphological operations are performed on the cracked portion to extract the crack skeleton line. Reference points for crack movement are defined on both sides of the skeleton line, and the crack development pattern is calculated based on the movement of the reference points. The calculation method achieves automated detection and high accuracy quantification of cracks in any direction during the loading of concrete beams, and is able to handle local rotation of the specimen, leveraging the full potential of DIC measurement technique. The validity and accuracy of the method are verified through four sets of laboratory shear tests of reinforced concrete beams. The results show that the method is capable of tracking and quantifying the crack development in all directions over the full range in real time during multiple loading stages.