A crack in a structure may be described using the local flexibility. The modal parameters will vary when a crack appears, and these changes may be used to identify its size and location. A method of pipe crack quantitative identification based on the change in local flexibility is set up. First, the principle of local flexibility due to the crack in a pipe may be researched by dividing a cracked pipe into a series of nested thin annuli in radial directions. Then, the influence function of the local flexibility and natural frequencies is obtained, and the surfaces of the natural frequency are drawn. Furthermore, the corresponding frequency contours may be obtained by using the first three order natural frequencies measured to cut the natural frequencies, and the size and location of the crack may be identified by the point of intersection. The effectiveness of this method is experimentally proven.