Abstract: An algorithm based on image processing for crack detection of expansion circle is proposed, which is named expansion circle, the crack is assumed to be a plane filled with several circular shapes, and the diameter of the circle can be approximated as the crack width. The geometrical model of the crack can be established and restored through a series of coordinates and diameters of the circle. The results show that after the parametrization of the crack, the image format is converted into the coordinates and diameters of several circles, and the main shape and characteristics of the crack are retained, which can be used as a further analysis operation for crack or structural health assessment. It provides a solution for crack image detection and structural health monitoring.