When a steel etc. a ferromagnetic material is subjected to the external loads, the internal force will develop, that is, stress will develop inside the material, which will lead to a corresponding change in the magnetic permeability of this material when magnetized. In this paper, according to this unique property of the ferromagnetic material, in the light of the mainly stressed and deformation characteristic of a long span steel structure etc. defect sensitive structures, by the use of stress sensitivity of magnetization, a mathematical physical model of relationship between the change in the magnetic permeability and stress is studied and put forward when a ferromagnetic component is under the action of a simple stress, a linear dependency relation of the component’s stress and its permeability is obtained, and the fundamental theory of the magnetic non-destructive testing of a safe state and its engineering application method is also established for a long span steel structure etc. defect sensitive structures, which provides a definite basis for guaranteeing safety of this kind of structures. Additionally, in this paper through the comparison of the theoretical analysis results with results of the related experiments, the dependency relation of change in the magnetic permeability and stress etc. theoretical research results is verified. Both these results exhibit a good consistence, which demonstrates that the research and the application of the non-destructive safe testing method for a defect sensitive structure based on the magnetic effect is feasible and valid and the future will see a certain engineering application prospect in this research and this testing method.