Abstract: To meet the need of developing the smart visualized virtual machine about alternating current field measurement technique, the characteristic signals of the electromagnetism are picked up among those disturbing electromagnetic signals. The information of the longth and depth of crack is described by three characteristic vectors. The numerical simulation is presented as the complement of experiments. Many kinds of distributions of the magnetic field above the arbitrary shape cracks are achieved by the simulating calculation. The influence of cracks shape on the precision in crack sizing is analyzed on the basis of those calculaion results, and the similarity in shape between the magnetic field signal and the crack is also detected by the simulating calculation. Furthermore, the inversion arithmetic of crack shape is proposed by studying the principle of the comparability, which has been proved by the experiment.