Abstract: In the light of the main stressed characteristics for a long-span steel structure, according to the stress dependent magnetization, a mathematical physical model involving the change in the magnetic permeability and stress was put forward when the ferromagnetic components under the action of the simple stresses, and the linear dependency relation was obtained, then the basic theory and application method of the magnetic NDT testing of a safety state for a long-span defect sensitive seracture was proposed. Through the related tests, it proved validity of the theory, and the method was effective and feasible.