Based on counter magnetostriction effect the magnetization intensity and stress sensitivity was analyzed for ferromagnetic material. The stress sensitivity of magnetized material was discussed on basis of the law of minimum energy and movement of magnetic domain structure. In accordance with the effect of magnetization intensity from stress the applied stress can lead to movement of domain wall of magnetic domain in the demagnetizing state for ferromagnetic material when the net magnetic moment is equal to zero. Meanwhile easily magnetization axis can be established on known applied stress. Then when ferromagnetic material is forced by the applied stress the magnetization intensity can be changed obviously. It is happened so that magnetic flux is also changed simultaneously. The result shows that real-time strained condition of material can be judged on basis of testing change of magnetic flux. Furthermore the study theory of magnetic nondestructive testing of defect sensibility structure for long span steel structural is obtained through this analysis.